清单大小错误

Question

我编写了从Firebase数据库获取密钥并将其写入ListView组件的代码。有一个我无法解决的简单问题。

我的代码如下。有人可以帮我吗？

List<String> list;
ArrayAdapter<String> adapter;
ListView rooms_list;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_rooms_list);

    rooms_list = findViewById(R.id.rooms_list);
    adapter = new ArrayAdapter<>(this, R.layout.rooms_list_layout, list);

    DatabaseReference mref = FirebaseDatabase.getInstance().getReference("chatRooms");
    mref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                list.add(snapshot.getKey());
                adapter.notifyDataSetChanged();
            }
        }

        @Override
        public void onCancelled(DatabaseError firebaseError) {

        }
    });

    rooms_list.setAdapter(adapter);


}

}