Crash when a nested widget is set to "Force Slow Construction Path"

Does not occur if the root widget also disables the fast path.

As a result, the current workaround is to ensure that the fast construction path is also disabled on the parent widget by making sure the "Force Slow Construction Path" option is turned on for both. Or vice-versa. In other words, the setting currently has to match between parent and child widgets - they either both have to disable the fast path, or both must enable it - in order to avoid hitting the assertion.

1. Create a new, blank project.
2. Add a new Widget Blueprint (NestWidget). Double-click to open it in the editor. Click "Class Settings" in the toolbar, then in the "Details" tab, check the "Force Slow Construction Path" box.
3. Compile, save and close the editor window.
4. Add another new Widget Blueprint (RootWidget). Double-click to open it in the editor. In the Palette tab search for 'NestWidget' and drag it into the Hierarchy.
5. Compile, save and close the editor window.
6. Open the current Level Blueprint and drag off the "BeginPlay" node to add a "CreateWidget" node. Set the 'Class' pin to "Root Widget" and the 'Owning Player' pin to the GetPlayerController function (Player Index 0).
7. Save the current level (NewMap).
8. Choose Edit->Project Settings->Maps and Modes and change the game map to 'NewMap'.
9. Choose File->Package Project->Windows->Win64 and wait for packaging to finish.
10. Attempt to launch the game. Note that an assertion will result:

[2018.05.30-06.24.18:167][  1]LogWindows: Error: === Critical error: ===
[2018.05.30-06.24.18:167][  1]LogWindows: Error: 
[2018.05.30-06.24.18:167][  1]LogWindows: Error: Assertion failed: InCanTemplate [File:E:\dev\UE4\Release-4.18\Engine\Source\Runtime\UMG\Private\WidgetBlueprintGeneratedClass.cpp] [Line: 128] 
[2018.05.30-06.24.18:167][  1]LogWindows: Error: 
[2018.05.30-06.24.18:167][  1]LogWindows: Error: 
[2018.05.30-06.24.18:167][  1]LogWindows: Error: 

[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff8d7c44008 KERNELBASE.dll!UnknownFunction []
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7694393cc UE4Game.exe!FWindowsErrorOutputDevice::Serialize() [d:\dev\ue4\dev-framework\engine\source\runtime\applicationcore\private\windows\windowserroroutputdevice.cpp:65]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7691a27fb UE4Game.exe!FOutputDevice::LogfImpl() [d:\dev\ue4\dev-framework\engine\source\runtime\core\private\misc\outputdevice.cpp:70]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff769135318 UE4Game.exe!FDebug::AssertFailed() [d:\dev\ue4\dev-framework\engine\source\runtime\core\private\misc\assertionmacros.cpp:421]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac16380 UE4Game.exe!UWidgetBlueprintGeneratedClass::InitializeWidgetStatic() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgetblueprintgeneratedclass.cpp:133]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac16141 UE4Game.exe!UWidgetBlueprintGeneratedClass::InitializeWidget() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgetblueprintgeneratedclass.cpp:247]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac15572 UE4Game.exe!UUserWidget::Initialize() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\userwidget.cpp:359]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76abfe710 UE4Game.exe!UWidgetTree::ForWidgetAndChildren() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgettree.cpp:181]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76abfe466 UE4Game.exe!UWidgetTree::ForEachWidget() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgettree.cpp:129]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac163bb UE4Game.exe!UWidgetBlueprintGeneratedClass::InitializeWidgetStatic() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgetblueprintgeneratedclass.cpp:148]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac16141 UE4Game.exe!UWidgetBlueprintGeneratedClass::InitializeWidget() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgetblueprintgeneratedclass.cpp:247]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac15572 UE4Game.exe!UUserWidget::Initialize() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\userwidget.cpp:359]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76abf4761 UE4Game.exe!UUserWidget::CreateWidgetOfClass() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\userwidget.cpp:1974]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76aba5349 UE4Game.exe!CreateWidget<UUserWidget>() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\public\blueprint\userwidget.h:1151]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76abf1868 UE4Game.exe!UWidgetBlueprintLibrary::Create() [d:\dev\ue4\dev-framework\engine\source\runtime\umg\private\widgetblueprintlibrary.cpp:54]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ac9fa34 UE4Game.exe!UWidgetBlueprintLibrary::execCreate() []
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7694f822b UE4Game.exe!UFunction::Invoke() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\class.cpp:4572]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696c260a UE4Game.exe!UObject::CallFunction() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:767]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696ddb0f UE4Game.exe!UObject::ProcessContextOpcode() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:2191]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696f26f1 UE4Game.exe!UObject::execLetObj() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:2011]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696e03c7 UE4Game.exe!UObject::ProcessInternal() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:978]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696c2f56 UE4Game.exe!UObject::CallFunction() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:890]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696e03c7 UE4Game.exe!UObject::ProcessInternal() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:978]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7694f822b UE4Game.exe!UFunction::Invoke() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\class.cpp:4572]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff7696df96f UE4Game.exe!UObject::ProcessEvent() [d:\dev\ue4\dev-framework\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1338]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76aeb949b UE4Game.exe!AActor::ProcessEvent() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\actor.cpp:730]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ae90341 UE4Game.exe!AActor::BeginPlay() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\actor.cpp:3237]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76ae982b1 UE4Game.exe!AActor::DispatchBeginPlay() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\actor.cpp:3197]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76bcf8b70 UE4Game.exe!AWorldSettings::NotifyBeginPlay() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\worldsettings.cpp:224]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76b3682d1 UE4Game.exe!AGameStateBase::HandleBeginPlay() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\gamestatebase.cpp:177]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76bce165e UE4Game.exe!UWorld::BeginPlay() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\world.cpp:3669]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76bbfdcc8 UE4Game.exe!UEngine::LoadMap() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\unrealengine.cpp:11839]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76bba08bd UE4Game.exe!UEngine::Browse() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\unrealengine.cpp:11095]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76b3840ba UE4Game.exe!UGameInstance::StartGameInstance() [d:\dev\ue4\dev-framework\engine\source\runtime\engine\private\gameinstance.cpp:485]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff768aa4f4c UE4Game.exe!FEngineLoop::Init() [d:\dev\ue4\dev-framework\engine\source\runtime\launch\private\launchengineloop.cpp:2788]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff768ab7932 UE4Game.exe!GuardedMain() [d:\dev\ue4\dev-framework\engine\source\runtime\launch\private\launch.cpp:155]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff768ab7a9a UE4Game.exe!GuardedMainWrapper() [d:\dev\ue4\dev-framework\engine\source\runtime\launch\private\windows\launchwindows.cpp:144]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff768ac5496 UE4Game.exe!WinMain() [d:\dev\ue4\dev-framework\engine\source\runtime\launch\private\windows\launchwindows.cpp:223]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff76cb4303b UE4Game.exe!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:253]
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff8d9561fe4 KERNEL32.DLL!UnknownFunction []
[2018.05.25-19.08.06:109][ 0]LogWindows: Error: [Callstack] 0x00007ff8dab3f061 ntdll.dll!UnknownFunction []