Error: Ensure condition failed: !IsGarbageCollecting()

At the End Play if I’m using SWebBrowser::LoadURL(FString NewURL) - I received:

LogOutputDevice: Error: Ensure condition failed: !IsGarbageCollecting() [File:D:/Build/++UE4/Sync/Engine/Source/Runtime/UMG/Private/Slate/SObjectWidget.cpp] [Line: 43] 
SObjectWidget for 'WBP_Promo_C /Engine/Transient.UnrealEdEngine_0:BP_MQGameInstance_C_1.WBP_Promo_C_7' destroyed while collecting garbage.  This can lead to multiple GCs being required to cleanup the object.  Possible causes might be,
1) ReleaseSlateResources not being implemented for the owner of this pointer.
2) You may just be holding onto some slate pointers on an actor that don't get reset until the actor is Garbage Collected.  You should avoid doing this, and instead reset those references when the actor is Destroyed.

I’m tried ReleaseSlateResources(bReleaseChildren) of the webbrowser widget but it’s not work.

How can I cleanup webbrowser refs before endplay?