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Crash occured when invoke AddToRoot and AddToViewport at one time

code to reproduce issue:

 UUserWidget* Widget = CreateWidget<UUserWidget>(MyPlayerController, WidgetTemplate);
 if (Widget)

If invoke AddToRoot and AddToViewport at the same time, UE4Editor would crash when click Stop button of Editor at runtime.

crash dump:

 Assertion failed: !IsRooted() [File:d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\coreuobject\public\uobject\UObjectBaseUtility.h] [Line: 135] 
 UE4Editor_Core!FDebug::AssertFailed() [d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:421]
 UE4Editor_UnrealEd!<lambda_26419e543909ee92ebfb672b1e9c08dc>::operator()() [d:\build\++ue4+release-4.13+compile\sync\engine\source\editor\unrealed\private\playlevel.cpp:384]
 UE4Editor_CoreUObject!ForEachObjectWithOuter() [d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjecthash.cpp:678]
 UE4Editor_UnrealEd!UEditorEngine::EndPlayMap() [d:\build\++ue4+release-4.13+compile\sync\engine\source\editor\unrealed\private\playlevel.cpp:380]
 UE4Editor_UnrealEd!UEditorEngine::Tick() [d:\build\++ue4+release-4.13+compile\sync\engine\source\editor\unrealed\private\editorengine.cpp:1653]
 UE4Editor_UnrealEd!UUnrealEdEngine::Tick() [d:\build\++ue4+release-4.13+compile\sync\engine\source\editor\unrealed\private\unrealedengine.cpp:371]
 UE4Editor!FEngineLoop::Tick() [d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\launch\private\launchengineloop.cpp:2834]
 UE4Editor!GuardedMain() [d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\launch\private\launch.cpp:156]
 UE4Editor!GuardedMainWrapper() [d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
 UE4Editor!WinMain() [d:\build\++ue4+release-4.13+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:202]
 UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:264]

I think UE4 should prevent users to invoke AddToRoot and AddToViewport at the same time, or give an exact tip to warn users from that usage.

Product Version: UE 4.13
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asked Oct 18 '16 at 04:21 PM in C++ Programming

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avatar image Doug E ♦♦ STAFF Oct 18 '16 at 04:46 PM


Where are you adding/calling this code? Is this happening inside an actor class, or in a begin play function, or a function being called later during runtime. Please include additional details to help me reproduce / investigate the behavior you're seeing.

avatar image 笑傲江湖-DA Oct 18 '16 at 05:30 PM

Hello, Doug Wilson. I call it in a Widget Button click event in a custom UUserWidget class. and I have several UUserWidget* need to create in my app.

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1 answer: sort voted first

The reason the editor is crashing on exit is because "Widget" is not being cleaned up when the object it is being added to is destroyed. Adding a call to Widget->RemoveFromRoot(); at any point before the object is destroyed should prevent the editor from crashing on exit.


Doug Wilson

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answered Oct 18 '16 at 06:40 PM

avatar image 笑傲江湖-DA Oct 19 '16 at 06:10 AM

Oh, I see, it's my wrong. Ty so much!

avatar image 笑傲江湖-DA Oct 19 '16 at 10:06 AM

Execuse me, I have another question, how can I get the event of Stop button of UE4Editor, this crash happens before the BeginDestroy of GameMode and UserWidget.

avatar image 笑傲江湖-DA Oct 19 '16 at 10:38 AM

Oh, I see, Event of Stop button of Editor is MyGameInstance::Shutdown(), not in GameMode.

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