java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAd

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAd 运行项目的时候,总是运行不出项目,debug之后,发现mPlaylist变量为null,没有将其实例化,加入了 mPlaylist = findViewById(R.id.lv_playlist);之后,运行成功。

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAd