Bdtjtk

i just have problem when i trying to logout from my app and close it and i open it again. The app was crashed there and i think that because onStart on my MainActivity.class not working , i spend lot of times to solve this but i can't , i need help so much.

This is the error

12-27 19:32:01.753 27788-27788/com.suryaaditya.dissent E/AndroidRuntime: FATAL EXCEPTION: main

Process: com.suryaaditya.dissent, PID: 27788

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference

    at com.suryaaditya.dissent.FriendsFragment.onCreateView(FriendsFragment.java:60)

    at android.support.v4.app.Fragment.performCreateView(Fragment.java:2346)

    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1428)

    at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759)

    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827)

    at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)

    at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2596)

    at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2383)

    at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)

    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2245)

    at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3248)

    at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3200)

    at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:195)

    at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:597)

    at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:177)

    at com.suryaaditya.dissent.MainActivity.onStart(MainActivity.java:113)

    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1392)

    at android.app.Activity.performStart(Activity.java:7157)

    at android.app.ActivityThread.handleStartActivity(ActivityThread.java:2940)

    at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180)

    at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165)

    at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142)

    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)

    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1811)

    at android.os.Handler.dispatchMessage(Handler.java:106)

    at android.os.Looper.loop(Looper.java:193)

    at android.app.ActivityThread.main(ActivityThread.java:6694)

    at java.lang.reflect.Method.invoke(Native Method)

    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)

    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

And this MainActivity

public class MainActivity extends AppCompatActivity {



private Toolbar mToolbar;

private ViewPager myViewPager;

private TabLayout myTabLayout;

private TabsAccessorAdapter myTabsAccessorAdapter;



private FirebaseAuth mAuth;

private DatabaseReference RootRef;



private String currentUserID;





@Override

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);



    mAuth = FirebaseAuth.getInstance();

    RootRef = FirebaseDatabase.getInstance().getReference();



    mToolbar = (Toolbar) findViewById(R.id.main_page_toolbar);

    setSupportActionBar(mToolbar);

    getSupportActionBar().setTitle("Dissent");



    BottomNavigationView navigationView= findViewById(R.id.bottom_navigation);

    final ChatsFragment chatsFragment = new ChatsFragment();

    final NotificationsFragment notificationsFragment = new NotificationsFragment();

    final FriendsFragment friendsFragment = new FriendsFragment();

    final GroupsFragment groupsFragment = new GroupsFragment();



    navigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {

        @Override

        public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {



            int id = menuItem.getItemId();

            if (id == R.id.navigation_friends){

                setFragment(friendsFragment);

                return true;

            }

            else if(id == R.id.navigation_group){

                setFragment(groupsFragment);

                return true;

            }

            else  if (id == R.id.navigation_chats){

                setFragment(chatsFragment);

                return true;

            }

            else if(id == R.id.navigation_notif){

                setFragment(notificationsFragment);

                return true;

            }



            return false;

        }

    });





    navigationView.setSelectedItemId(R.id.navigation_friends);

}



private  void setFragment(Fragment fragment){

    FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();

    fragmentTransaction.replace(R.id.frame, fragment);

    fragmentTransaction.commit();

}



@Override

protected void onStart() {



    super.onStart();



    FirebaseUser currentUser = mAuth.getCurrentUser();



    if (currentUser == null){

        SendUserToLoginActivity();

    }

    else{

        updateUserStatus("online");

        VerifyUserExistence();

    }

}



@Override

protected void onStop() {

    super.onStop();





    FirebaseUser currentUser = mAuth.getCurrentUser();





    if (currentUser != null){



        updateUserStatus("offline");

    }



}



@Override

protected void onDestroy() {

    super.onDestroy();





    FirebaseUser currentUser = mAuth.getCurrentUser();





    if (currentUser != null){



        updateUserStatus("offline");

    }

}



private void VerifyUserExistence() {



    String currentUserID = mAuth.getCurrentUser().getUid();



    RootRef.child("Users").child(currentUserID).addValueEventListener(new ValueEventListener() {

        @Override

        public void onDataChange(DataSnapshot dataSnapshot) {

            if ((dataSnapshot.child("name").exists())){



            }

            else {

                SendUserToSettingsActivity();

            }

        }



        @Override

        public void onCancelled(DatabaseError databaseError) {



        }

    });



}



private void SendUserToLoginActivity() {

    Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);

    loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);

    startActivity(loginIntent);

}



private void SendUserToFindFriendsActivity() {

    Intent findFriendsIntent = new Intent(MainActivity.this , FindFriendsActivity.class);

    startActivity(findFriendsIntent);

}



private void SendUserToSettingsActivity() {

    Intent settingsIntent = new Intent(MainActivity.this , SettingsActivity.class);

    startActivity(settingsIntent);

}







@Override

public boolean onCreateOptionsMenu(Menu menu) {



    super.onCreateOptionsMenu(menu);



    getMenuInflater().inflate(R.menu.option_menu, menu);



    return true;

}



@Override

public boolean onOptionsItemSelected(MenuItem item) {

    super.onOptionsItemSelected(item);



    if (item.getItemId() == R.id.main_logout_option){



        updateUserStatus("offline");



        mAuth.signOut();

        SendUserToLoginActivity();



    }



    if (item.getItemId() == R.id.main_settings_option){



        SendUserToSettingsActivity();



    }



    if (item.getItemId() == R.id.main_create_group_option){



        RequestNewGroup();



    }



    if (item.getItemId() == R.id.main_find_friends_option){

        SendUserToFindFriendsActivity();

    }



    return true;

}



private void RequestNewGroup() {



    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this, R.style.AlertDialog);

    builder.setTitle("Enter Group Name : ");

    final EditText groupNameField = new EditText(MainActivity.this);

    groupNameField.setHint("e.g Dissent");

    builder.setView(groupNameField);



    builder.setPositiveButton("Create", new DialogInterface.OnClickListener() {

        @Override

        public void onClick(DialogInterface dialog, int which) {



            String groupName = groupNameField.getText().toString();



            if (TextUtils.isEmpty(groupName)){



                Toast.makeText(MainActivity.this, "Please enter Group Name !" , Toast.LENGTH_SHORT).show();



            }

            else {



                CreateNewGroup(groupName);



            }



        }

    });



    builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() 

{

        @Override

        public void onClick(DialogInterface dialog, int which) {



            dialog.cancel();



        }

    });



    builder.show();



}



private void CreateNewGroup(final String groupName) {



    RootRef.child("Groups").child(groupName).setValue("")

            .addOnCompleteListener(new OnCompleteListener<Void>() {

                @Override

                public void onComplete(@NonNull Task<Void> task) {

                    if (task.isSuccessful()){

                        Toast.makeText(MainActivity.this, groupName + " is Created Successfully !", Toast.LENGTH_SHORT).show();

                    }

                }

            });



}



private void updateUserStatus(String state){



    String saveCurrentTime, saveCurrentDate;



    Calendar calendar = Calendar.getInstance();



    SimpleDateFormat currentTime = new SimpleDateFormat("hh:mm a");

    saveCurrentTime = currentTime.format(calendar.getTime());



    SimpleDateFormat currentDate = new SimpleDateFormat("MMM dd, yyyy");

    saveCurrentDate = currentDate.format(calendar.getTime());





    HashMap<String , Object> onlineStateMap = new HashMap<>();

    onlineStateMap.put("time", saveCurrentTime);

    onlineStateMap.put("date", saveCurrentDate);

    onlineStateMap.put("state", state);





    currentUserID = mAuth.getCurrentUser().getUid();

    RootRef.child("Users").child(currentUserID).child("userState")

            .updateChildren(onlineStateMap);

}

}

This is FriendsFragment

public class FriendsFragment extends Fragment {



private View FriendsView;

private RecyclerView friendsList ;



private DatabaseReference friendsRef, usersRef ;

private FirebaseAuth mAuth;



private String currentUserID;



public FriendsFragment() {

    // Required empty public constructor

}





@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container,

                         Bundle savedInstanceState) {

    // Inflate the layout for this fragment

    FriendsView = inflater.inflate(R.layout.fragment_friends, container, false);





    friendsList = (RecyclerView) FriendsView.findViewById(R.id.friends_list);

    friendsList.setLayoutManager(new LinearLayoutManager(getContext()));



    mAuth = FirebaseAuth.getInstance();

    currentUserID = mAuth.getCurrentUser().getUid();



    friendsRef = FirebaseDatabase.getInstance().getReference().child("Contacts").child(currentUserID);

    usersRef = FirebaseDatabase.getInstance().getReference().child("Users");



    return FriendsView;

}



@Override

public void onStart() {

    super.onStart();



    FirebaseRecyclerOptions options =

            new FirebaseRecyclerOptions.Builder<Contacts>()

            .setQuery(friendsRef, Contacts.class)

            .build();



    FirebaseRecyclerAdapter<Contacts, FriendsViewHolder> adapter =

            new FirebaseRecyclerAdapter<Contacts, FriendsViewHolder>(options) 

{

                @Override

                protected void onBindViewHolder(@NonNull final FriendsViewHolder holder, int position, @NonNull Contacts model) {



                    String userID = getRef(position).getKey();



                    usersRef.child(userID).addValueEventListener(new ValueEventListener() {

                        @Override

                        public void onDataChange(DataSnapshot dataSnapshot) {



                            if (dataSnapshot.exists()){



                                if (dataSnapshot.child("userState").hasChild("state")) {



                                    String state = dataSnapshot.child("userState").child("state").getValue().toString();

                                    String date = dataSnapshot.child("userState").child("date").getValue().toString();

                                    String time = dataSnapshot.child("userState").child("time").getValue().toString();



                                    if (state.equals("online")){



                                        holder.onlineIcon.setVisibility(View.VISIBLE);

                                    }

                                    else if (state.equals("offline")){



                                        holder.onlineIcon.setVisibility(View.INVISIBLE);

                                    }

                                }

                                else {



                                    holder.onlineIcon.setVisibility(View.INVISIBLE);

                                }



                                if (dataSnapshot.hasChild("image")){



                                    String userImage = dataSnapshot.child("image").getValue().toString();

                                    String profileName = dataSnapshot.child("name").getValue().toString();

                                    String profileStatus = dataSnapshot.child("status").getValue().toString();





                                    holder.userName.setText(profileName);

                                    holder.userStatus.setText(profileStatus);

                                    Picasso.get().load(userImage).placeholder(R.drawable.profile_image).into(holder.profileImage);



                                }

                                else {



                                    String profileName = dataSnapshot.child("name").getValue().toString();

                                    String profileStatus = dataSnapshot.child("status").getValue().toString();



                                    holder.userName.setText(profileName);

                                    holder.userStatus.setText(profileStatus);

                                }

                            }



                        }



                        @Override

                        public void onCancelled(DatabaseError databaseError) {



                        }

                    });



                }



                @NonNull

                @Override

                public FriendsViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int viewType) {



                    View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.users_display_layout, viewGroup,false);



                    FriendsViewHolder viewHolder = new FriendsViewHolder(view);

                    return viewHolder;



                }

            };



        friendsList.setAdapter(adapter);

        adapter.startListening();



}



public static class FriendsViewHolder extends RecyclerView.ViewHolder{



    TextView userName, userStatus;

    CircleImageView profileImage;

    ImageView onlineIcon;



    public FriendsViewHolder(View itemView) {

        super(itemView);



        userName = itemView.findViewById(R.id.user_profile_name);

        userStatus = itemView.findViewById(R.id.user_status);

        profileImage = itemView.findViewById(R.id.user_profile_image);

        onlineIcon = (ImageView) itemView.findViewById(R.id.user_online_status);



    }

}

}

The problem is on FriendsFragment. You are trying to get Uid on a null object reference which means that the user isnt logged in when you transact to FriendsFragment.