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

Question

My app was working perfectly before until this morning when I ran my code in android studio and it crashed, I checked logcat and it pointed to these two places

first is in the MainActivity.java in the onStart

  mAuth = FirebaseAuth.getInstance();


    mToolbar = findViewById(R.id.main_app_bar);
    setSupportActionBar(mToolbar);
    getSupportActionBar().setTitle("ChatPlus");


    BottomNavigationView navigation = findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);

    loadFragment(new RequestFragment());


    if (mAuth.getCurrentUser() != null) {
        mUserRef = FirebaseDatabase.getInstance().getReference().child("Users").child(mAuth.getCurrentUser().getUid());

    }
}


@Override
public void onStart() {
    super.onStart(); //crashes here
    // Check if user is signed in (non-null) and update UI accordingly.
    FirebaseUser currentUser = mAuth.getCurrentUser();

    if (currentUser == null) {
        sendtostart();
    } else {
        mUserRef.child("online").setValue("true");
    }
}

@Override
protected void onStop() {
    super.onStop();

    FirebaseUser currentUser = mAuth.getCurrentUser();

    if (currentUser != null) {
        mUserRef.child("online").setValue(ServerValue.TIMESTAMP);
    }
}

second place is in my RequestFragment

 mMainView = inflater.inflate(R.layout.fragment_request, container, false);

    mRequestList = (RecyclerView) mMainView.findViewById(R.id.request_list);
    mAuth = FirebaseAuth.getInstance();

    mCurrent_user_id = mAuth.getCurrentUser().getUid(); //at this line


    mRequestDatabase = FirebaseDatabase.getInstance().getReference().child("Friend_req").child(mCurrent_user_id);
    mRequestDatabase.keepSynced(true);
    mUsersDatabase = FirebaseDatabase.getInstance().getReference().child("Users");
    mUsersDatabase.keepSynced(true);