Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.graphics.drawable.AnimationDrawable.start()' on a null object reference

[finddreams]

if (mIvLoading!=null) { ((AnimationDrawable) mIvLoading.getDrawable()).start(); }
换肤后这样的代码会抛异常崩溃，是什么原因？