title: Android app crashes from time to time after App was in background for a while, in ViewPager.getChildDrawingOrder
body: <pre><code>Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.ArrayList.get(int)' on a null object reference
     at android.support.v4.view.ViewPager.getChildDrawingOrder(ViewPager.java:798)
     at android.view.ViewGroup.getAndVerifyPreorderedIndex(ViewGroup.java:1986)
     at android.view.ViewGroup.populateChildrenForAutofill(ViewGroup.java:3712)
     at android.view.ViewGroup.getChildrenForAutofill(ViewGroup.java:3698)
     at android.view.ViewGroup.dispatchProvideAutofillStructure(ViewGroup.java:3680)
     at android.view.ViewGroup.dispatchProvideAutofillStructure(ViewGroup.java:3686)
     at android.view.ViewGroup.dispatchProvideAutofillStructure(ViewGroup.java:3686)
     at android.view.ViewGroup.dispatchProvideAutofillStructure(ViewGroup.java:3686)
     at android.view.ViewGroup.dispatchProvideAutofillStructure(ViewGroup.java:3686)
     at android.app.assist.AssistStructure$WindowNode.&lt;init&gt;(AssistStructure.java:510)
     at android.app.assist.AssistStructure.&lt;init&gt;(AssistStructure.java:1912)
     at android.app.ActivityThread.handleRequestAssistContextExtras(ActivityThread.java:3175)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1913)
     at android.os.Handler.dispatchMessage(Handler.java:105)
     at android.os.Looper.loop(Looper.java:164)
     at android.app.ActivityThread.main(ActivityThread.java:6944)
     at java.lang.reflect.Method.invoke(Method.java)
     at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
  </code></pre>