[Android] Fix potential NPE

[YorkShen]

java.lang.RuntimeException: android.os.DeadSystemException
at android.app.ActivityThread.performPauseActivityIfNeeded(ActivityThread.java:4103)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:4069)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:4043)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:4017)
at android.app.ActivityThread.-wrap18(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1640)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6646)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
Caused by: java.lang.RuntimeException: android.os.DeadSystemException
at android.app.ContextImpl.sendBroadcast(ContextImpl.java:915)
at android.content.ContextWrapper.sendBroadcast(ContextWrapper.java:452)
at com.taobao.weex.WXSDKInstance.onActivityPause(WXSDKInstance.java:1324)