Java RecyclerView未显示firestore中的任何数据

我在firestore数据库中有主题类别列表，我希望它显示在我的应用程序中（我认为可以通过recyclerView完成）。但我的recyclerView没有填充任何数据。你可以在下面看到我的代码：

Question_list.java

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import com.google.firebase.firestore.DocumentChange;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.google.firebase.firestore.QuerySnapshot;

import java.util.ArrayList;
import java.util.List;

import javax.annotation.Nullable;

public class Question_list extends AppCompatActivity {

private static final String TAG ="Firelog" ;
private RecyclerView mMainList;
private FirebaseFirestore mFirestore;
private CategoryAdapter categoryAdapter;
private List<Categories> list_category;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_question_list);

    mMainList = (RecyclerView)findViewById(R.id.main_list_id);
    //mMainList.setHasFixedSize(true);
    mMainList.setLayoutManager(new LinearLayoutManager(this));
    mMainList.setAdapter(categoryAdapter);
    mFirestore = FirebaseFirestore.getInstance();
    categoryAdapter = new CategoryAdapter(list_category);
    list_category = new ArrayList<>();

    mFirestore.collection("Questions").addSnapshotListener(new EventListener<QuerySnapshot>() {
        @Override
        public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {

            if(e!=null){
                Log.d(TAG, "Error: "+ e.getMessage());
            }
            for(DocumentChange doc: queryDocumentSnapshots.getDocumentChanges()){
                if(doc.getType() == DocumentChange.Type.MODIFIED){
                    //String document = doc.getDocument().getString("Category");

                    Categories categories = doc.getDocument().toObject(Categories.class);
                    list_category.add(categories);
                    Log.d(TAG, "Category: "+ categories);
                    categoryAdapter.notifyDataSetChanged();
                }
            }
        }
    });

}
}

以上是我所有的java类。我还没有上传我的XML文件，因为我认为问题出在java类中。

categoryAdapter=newcategoryadapter（list\u category）；
categoryAdapter = new CategoryAdapter(list_category);
list_category = new ArrayList<>();

list_category=new ArrayList（）；

列表类别应在将其传递给适配器之前初始化

list_category = new ArrayList<>();
categoryAdapter = new CategoryAdapter(list_category);

list_category=new ArrayList（）；
categoryAdapter=新的categoryAdapter（列表\类别）；

categoryAdapter = new CategoryAdapter(list_category);
list_category = new ArrayList<>();

list_category = new ArrayList<>();
categoryAdapter = new CategoryAdapter(list_category);