Creating a Blueprint Variable for InputActionKeyMappings, and then removing the default input parameter causes crash.

Upon creation and compilation of a Blueprint Variable of type InputActionKeyMappings, when clicking the "X" to remove the default input key parameter causes the editor to crash.

1. Launch Unreal Engine 4.21
2. Open up the default Level Blueprint
3. Create a new Variable "TestVariable"
4. Change the TestVariable's type to be of InputActionKeyMapping.
5. Compile the Blueprint
6. In the default value of TestVariable, select the X to remove the action mapping.

Result: Unreal Engine crashes

Expected:  Warning, or removing the default action mapping is unavailable to click.

Assertion failed: IsValid() [Link Removed] [Line: 849] UE4Editor_Core!FDebug::AssertFailed() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:417] UE4Editor_DetailCustomizations!FInputActionMappingCustomization::RemoveActionMappingButton_OnClick() [d:\build\++ue4\sync\engine\source\editor\detailcustomizations\private\inputstructcustomization.cpp:165] UE4Editor_DetailCustomizations!TBaseSPMethodDelegateInstance<0,FInputActionMappingCustomization,0,void _cdecl(void)>::ExecuteIfSafe() [d:\build\++ue4\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:355] UE4Editor_PropertyEditor!PropertyCustomizationHelpers::SPropertyEditorButton::OnClick() [d:\build\++ue4\sync\engine\source\editor\propertyeditor\private\propertycustomizationhelpers.cpp:81] UE4Editor_PropertyEditor!TBaseSPMethodDelegateInstance<0,PropertyCustomizationHelpers::SPropertyEditorButton,0,FReply __cdecl(void)>::Execute() [d:\build\++ue4\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:279] UE4Editor_Slate!SButton::OnMouseButtonUp() [d:\build\++ue4\sync\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:290] UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_1002768c627006711ef2f351a87ec0e7> >() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:270] UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5528] UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:6087] UE4Editor_Slate!FSlateApplication::OnMouseUp() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:6060] UE4Editor_ApplicationCore!FWindowsApplication::ProcessDeferredMessage() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:1831] UE4Editor_ApplicationCore!FWindowsApplication::DeferMessage() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:2281] UE4Editor_ApplicationCore!FWindowsApplication::ProcessMessage() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:929] UE4Editor_ApplicationCore!FWindowsApplication::AppWndProc() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:766] user32 user32 UE4Editor_ApplicationCore!FWindowsPlatformApplicationMisc::PumpMessages() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsplatformapplicationmisc.cpp:130] UE4Editor!FEngineLoop::Tick() [d:\build\++ue4\sync\engine\source\runtime\launch\private\launchengineloop.cpp:3615] UE4Editor!GuardedMain() [d:\build\++ue4\sync\engine\source\runtime\launch\private\launch.cpp:174] UE4Editor!GuardedMainWrapper() [d:\build\++ue4\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:145] UE4Editor!WinMain() [d:\build\++ue4\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:276] UE4Editor!_scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:283] kernel32 ntdll