Crash on UObject::ProcessEvent after 4.7 update

Crash Report
[Link Removed]
Bugg 38910

After updating project from 4.6.1 to 4.7, user experiences unexplained crash when playing in editor.

*In addtion to the crash report, the user provides a lot of additional information and screenshots on the forum post listed in "additional URL."

Do not have project files to reproduce locally.

<unknown module>! KERNELBASE!RaiseException()
<unknown module>! UE4Editor_Core!FOutputDeviceWindowsError::Serialize()
<unknown module>! UE4Editor_Core!FOutputDevice::Logf__VA()
<unknown module>! UE4Editor_Core!FDebug::AssertFailed()
<unknown module>! UE4Editor_CoreUObject!UObject::ProcessEvent()
<unknown module>! UE4Editor_UMG!UUserWidget::OnMouseLeave()
<unknown module>! UE4Editor_Slate!FSlateApplication::ProcessMouseMoveEvent()
<unknown module>! UE4Editor_Slate!FSlateApplication::SynthesizeMouseMove()
<unknown module>! UE4Editor_Slate!FSlateApplication::Tick()
<unknown module>! UE4Editor_UnrealEd!TickSlate()
<unknown module>! UE4Editor_UnrealEd!FFeedbackContextEditor::StartSlowTask()
<unknown module>! UE4Editor_Core!FScopedSlowTask::MakeDialog()
<unknown module>! UE4Editor_Engine!FStaticMeshStatusMessageContext::FStaticMeshStatusMessageContext()
<unknown module>! UE4Editor_Engine!FStaticMeshRenderData::Cache()
<unknown module>! UE4Editor_Engine!UStaticMesh::CacheDerivedData()
<unknown module>! UE4Editor_Engine!UStaticMesh::PostLoad()
<unknown module>! UE4Editor_CoreUObject!UObject::ConditionalPostLoad()
<unknown module>! UE4Editor_CoreUObject!FAsyncPackage::PostLoadObjects()
<unknown module>! UE4Editor_CoreUObject!FAsyncPackage::Tick()
<unknown module>! UE4Editor_CoreUObject!ProcessAsyncLoading()
<unknown module>! UE4Editor_CoreUObject!StaticTick()
<unknown module>! UE4Editor_UnrealEd!UEditorEngine::Tick()
<unknown module>! UE4Editor_UnrealEd!UUnrealEdEngine::Tick()
<unknown module>! UE4Editor!FEngineLoop::Tick()
<unknown module>! UE4Editor!GuardedMain()
<unknown module>! UE4Editor!GuardedMainWrapper()
<unknown module>! UE4Editor!WinMain()
<unknown module>! UE4Editor!__tmainCRTStartup()