如何修复“无法为数据库提供空上下文”-Android Room

Question

我正在尝试建立一个Room数据库，但是它说Context是空的，即使我在不​​同的应用程序中尝试了相同的类，它也可以正常工作。

@Database(entities = {City.class, User.class}, version = AppDatabase.DB_VERSION, exportSchema = false)
public abstract class AppDatabase extends RoomDatabase {

    static final int DB_VERSION = 1;
    private static AppDatabase INSTANCE;
    private static final String DATABASE_NAME = "my_db";

    public abstract UserDao userDao();
    public abstract CityDao cityDao();

    public static AppDatabase getInstance() {
        if (INSTANCE == null) {
            INSTANCE = Room.databaseBuilder(RoomApp.getAppContext(), AppDatabase.class, DATABASE_NAME)
                    .allowMainThreadQueries()
                    .build();
        }

        return INSTANCE;
    }
}

public class RoomApp extends Application {
    private static WeakReference<RoomApp> appContext;

    @Override
    public void onCreate() {
        super.onCreate();
        appContext = new WeakReference<>(this);

        AppDatabase.getInstance(); //this will init & fill db
    }

    public static Context getAppContext() {
        return appContext != null ? appContext.get() : null;
    }
}

每当我访问AppDatabase.getInstance（）;我收到一条错误消息：“无法为数据库提供空上下文。”