出现错误：
E/AndroidRuntime: FATAL EXCEPTION: Timer-1
Process: yanlan.example.caculate_weixin, PID: 9029
java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.drawable.ClipDrawable.getLevel()' on a null object reference
at yanlan.example.caculate_weixin.MainActivity$1.run(MainActivity.java:52)
at java.util.TimerThread.mainLoop(Timer.java:562)
at java.util.TimerThread.run(Timer.java:512)
I/Process: Sending signal. PID: 9029 SIG: 9
drawable为空，查看clip.xml文件设置的位置，应该在src上，不是在background上
android:src="@drawable/clip"
