[Gutenberg] Crash in image upload reattach

[mzorz]

A reproduceable crash happens in current develop after the introduction of 0e2237f in #9030 and media upload progress reattach

2019-02-18 15:08:11.402 32225-32225/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.wordpress.android, PID: 32225
    java.lang.IllegalStateException: Fragment has not been attached yet.
        at android.support.v4.app.Fragment.instantiateChildFragmentManager(Fragment.java:2300)
        at android.support.v4.app.Fragment.getChildFragmentManager(Fragment.java:763)
        at org.wordpress.android.editor.GutenbergEditorFragment.getGutenbergContainerFragment(GutenbergEditorFragment.java:91)
        at org.wordpress.android.editor.GutenbergEditorFragment.onMediaUploadReattached(GutenbergEditorFragment.java:572)
        at org.wordpress.android.ui.posts.EditPostActivity.reattachUploadingMedia(EditPostActivity.java:730)
        at org.wordpress.android.ui.posts.EditPostActivity.access$4100(EditPostActivity.java:192)
        at org.wordpress.android.ui.posts.EditPostActivity$SectionsPagerAdapter.instantiateItem(EditPostActivity.java:2209)
        at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1004)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1152)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1086)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1616)
        at android.view.View.measure(View.java:22071)
        at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
        at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
        at android.view.View.measure(View.java:22071)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6602)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
        at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:141)
        at android.view.View.measure(View.java:22071)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6602)
        at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:400)
        at android.view.View.measure(View.java:22071)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6602)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
        at android.view.View.measure(View.java:22071)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6602)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1514)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:806)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:685)
        at android.view.View.measure(View.java:22071)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6602)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:724)
        at android.view.View.measure(View.java:22071)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2422)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1504)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1761)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1392)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6752)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
        at android.view.Choreographer.doCallbacks(Choreographer.java:723)
        at android.view.Choreographer.doFrame(Choreographer.java:658)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
2019-02-18 15:08:11.409 1088-23124/? W/ActivityManager:   Force finishing activity org.wordpress.android/.ui.posts.EditPostActivity

Steps to reproduce the behavior

Follow steps described in #9129

1. start a new draft
2. insert an image
3. once it starts uploading, exit the editor
4. while still uploading, open the Post in the editor again
5. observe crash

Tested on [Pixel 2], Android [8.1]