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

` 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？