Проблема с nullpointer exeption c Room database android

myAppDatabase = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "notepadsdb") 
                    .allowMainThreadQueries() 
                    .build();

 private void fill_folder_array() { 
        Thread th1; 
        th1 = new Thread(new Runnable() { 
            @Override 
            public void run() { 
                folders = MainActivity.myAppDatabase.folderDao().getAllFolders(); 
            } 
        }); 
        th1.start(); 
 
        try{ 
        category_names.clear(); 
        category_ids.clear(); 
            th1.join(); 
            category_names.add("None"); 
            category_ids.add(-1); 
            for(int i = 0; i< folders.size(); i++){ 
                category_names.add(folders.get(i).getName()); 
                category_ids.add(folders.get(i).getId()); 
            } 
        } catch (InterruptedException e) { 
            e.printStackTrace(); 
        } 
    }