当我按下后退按钮，它是移动到上一个片段，但也显示一个空白屏幕，我需要点击它来消除它。我该如何摆脱这个空白屏幕？我看到它和第一个有某种联系 addToBackStack 但我还是修不好
这是我的 onNavigationItemSelected 方法：

public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        Fragment fragment=null;
        switch (menuItem.getItemId()) {
            case R.id.nav_home:
                fragment=new HomeFragment();
                break;
            case R.id.nav_income:
                fragment=new IncomeFragment();
                break;
            case R.id.nav_expense:
                fragment=new ExpenseFragment();
                break;
            case R.id.nav_sign_out:
                FirebaseAuth mAuth = FirebaseAuth.getInstance();
                mAuth.signOut();
                startActivity(new Intent(getApplicationContext(), WelcomeActivity.class));
                Toast.makeText(getApplicationContext(), "Signed Out", Toast.LENGTH_LONG).show();
                break;

        }
        if(fragment!=null)
            getSupportFragmentManager().beginTransaction().
                    replace(R.id.fragment_container, fragment).addToBackStack("").commit();

        return true;
    }

这是我的 onBackPressed() 方法：

public void onBackPressed() {
        final FragmentManager fragmentManager = getSupportFragmentManager();
        if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
            drawerLayout.closeDrawer(GravityCompat.START);
        } else {
            AlertDialog.Builder builder = new AlertDialog.Builder(this, R.style.AlertDialogCustom);
            if (fragmentManager.getBackStackEntryCount() == 0) {
                builder.setTitle("Sign out");
                builder.setMessage("Are you sure you want to sign out?");
                builder.setPositiveButton("YES", new DialogInterface.OnClickListener() {

                    public void onClick(DialogInterface dialog, int which) {
                        firebaseAuth.signOut();
                        dialog.dismiss();
                        startActivity(new Intent(getApplicationContext(), WelcomeActivity.class));
                    }
                });

                builder.setNegativeButton("NO", new DialogInterface.OnClickListener() {

                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.dismiss();
                    }
                });
            } else {
                    fragmentManager.popBackStack();
            }

            AlertDialog alert = builder.create();
            alert.show();
        }
    }