The second navigation to a fragment with a ViewPager throws an exception

[vecalion]

When a user the second time navigates to a fragment with a ViewPager, an exception is thrown:

Java.Lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/data/lanes/3053/a94a03b5/source/mono/external/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143 at Android.Runtime.JNIEnv.CallObjectMethod (IntPtr jobject, IntPtr jmethod, Android.Runtime.JValue* parms) [0x00064] in /Users/builder/data/lanes/3053/a94a03b5/source/monodroid/src/Mono.Android/src/Runtime/JNIEnv.g.cs:195 at Android.Support.V4.App.FragmentManagerInvoker.GetFragment (Android.OS.Bundle bundle, System.String key) [0x00064] in :0 at MvvmCross.Droid.Support.V4.MvxFragmentPagerAdapter2.RestoreState (IParcelable state, Java.Lang.ClassLoader loader) [0x000bd] in :0 at Android.Support.V4.View.PagerAdapter.n_RestoreState_Landroid_os_Parcelable_Ljava_langClassLoader (IntPtr jnienv, IntPtr native__this, IntPtr native_state, IntPtr native_loader) [0x00019] in :0 at at (wrapper dynamic-method) System.Object:60a811d7-76a2-41aa-a8a0-ff5c5f50127d (intptr,intptr,intptr,intptr) at --- End of managed exception stack trace --- at java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference at at android.support.v4.app.FragmentManagerImpl.getFragment(FragmentManager.java:667) at at md559873f5c332c022bf8710d3c9fbb7bb2.MvxFragmentPagerAdapter2.n_restoreState(Native Method) at at md559873f5c332c022bf8710d3c9fbb7bb2.MvxFragmentPagerAdapter2.restoreState(MvxFragmentPagerAdapter2.java:74) at at android.support.v4.view.ViewPager.onRestoreInstanceState(ViewPager.java:1319) at at android.view.View.dispatchRestoreInstanceState(View.java:13740) at at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:2887) at at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:2893) at at android.view.View.restoreHierarchyState(View.java:13718) at at android.support.v4.app.Fragment.restoreViewState(Fragment.java:468) at at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1094) at at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248) at at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738) at at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613) at at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:570) at at mono.android.view.View_OnClickListenerImplementor.n_onClick(Native Method) at at mono.android.view.View_OnClickListenerImplementor.onClick(View_OnClickListenerImplementor.java:29) at at android.view.View.performClick(View.java:4780) at at android.view.View$PerformClick.run(View.java:19866) at at android.os.Handler.handleCallback(Handler.java:739) at at android.os.Handler.dispatchMessage(Handler.java:95) at at android.os.Looper.loop(Looper.java:135) at at android.app.ActivityThread.main(ActivityThread.java:5254) at at java.lang.reflect.Method.invoke(Native Method) at at java.lang.reflect.Method.invoke(Method.java:372) at at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) at at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Sample project can be found here: https://github.com/vecalion/MvvmCross-Samples/tree/viewpager_crash

Steps to reproduce

1. Open XPlatformMenus project
2. Login and click on 'Show' button on the home page
3. Click hardware back button and click 'Show' button again.

The exception is thrown by this call: https://github.com/MvvmCross/MvvmCross-AndroidSupport/blob/master/MvvmCross.Droid.Support.V4/MvxFragmentPagerAdapter2.cs#L158

Suppressing the exception with try catch might be used as work-around, but what causes the issue?

[nmilcoff]

I can easily reproduce this issue in an app bumped from MvvmCross 4.0 to latest version (it works ok in 4.0)

[martijn00]

Please check again in the next release