当我尝试测试我的应用程序时遇到问题，我是 Android Studio 的初学者并试图将 reyclerview 放在片段中，但是当我编译时，我收到此消息：
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference
这是导致原因的文件：
package e.user.popcorn;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.MenuItem;
import android.widget.FrameLayout;
import android.widget.TextView;
import android.widget.Toolbar;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private BottomNavigationView mBottomNavigation;
private FrameLayout mMainFrame;
private HomeFragment homeFragment;
private MoviesFragment moviesFragment;
private UsersFragment usersFragment;
private ChatFragment chatFragment; …