我在運(yùn)行我的應(yīng)用程序時遇到了這個問題：嘗試在空對象引用上調(diào)用虛擬方法 java.lang.String com.google.firebase.auth.FirebaseUser.getDisplayName()我想制作一個用戶無需先登錄即可瀏覽的應(yīng)用程序。如何將name.setText("hi, " + currentUser.getDisplayName());if 語句放在里面 我應(yīng)該在 if 語句的編碼中寫什么？這是我的MainActivityjava類：@Overrideprotected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    name=findViewById(R.id.name);    //        //display customer's account name    name.setText("hi, " + currentUser.getDisplayName());    //ini    firebaseAuth = FirebaseAuth.getInstance();    currentUser = firebaseAuth.getCurrentUser();    databaseReference = FirebaseDatabase.getInstance().getReference("Cust");@Overridepublic boolean onCreateOptionsMenu(Menu menu) {    MenuInflater inflater = getMenuInflater();    inflater.inflate(R.menu.menu_option,menu);    return true;}@Overridepublic boolean onOptionsItemSelected(@NonNull MenuItem item) {    int id= item.getItemId();    if (id == R.id.logout){        logout();        return true;    }    else if (id==R.id.login){        login();        return true;    }    return super.onOptionsItemSelected(item);}private void login() {    Intent intent= new Intent(this,LoginActivity.class);    startActivity(intent);}private void logout() {    FirebaseAuth.getInstance().signOut();    Intent intent = new Intent(MainActivity.this, LoginActivity.class);    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);    startActivity(intent);    finish();}}