Crash when Enabling HDR in a Standalone Fullscreen Game

Enabling HDR with the r.HDR.EnableHDROutput command causes a crash in fullscreen standalone as well as packaged projects. This only happens with HDR monitors, following the same steps on a monitor that does not support HDR does not result in a crash.

1. Ensure HDR is enabled on the Monitor that will be displaying the project
2. Run any project in standalone mode
3. Press ALT+Enter to fullscreen the standalone window
4. Enable HDR with r.HDR.EnableHDROutput 1 from the console

Results: The Editor will crash in one of two ways. It will crash immediately or it will crash when trying to close the standalone window.

Expected: HDR to be enabled in the standalone window.

Assertion failed: IsInRenderingThread() [Link Removed] [Line: 1197]

UE4Editor_Core!FWindowsErrorOutputDevice::Serialize() [d:\build\++ue4\sync\engine\source\runtime\core\private\windows\windowserroroutputdevice.cpp:79]
UE4Editor_Core!FOutputDevice::LogfImpl() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:71]
UE4Editor_Core!FDebug::AssertFailed() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:440]
UE4Editor_Core!FDebug::CheckVerifyFailedImpl() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:418]
UE4Editor_RenderCore!DispatchCheckVerify<void,<lambda_7a79960a656aef26fac66632da9570c6> >() [d:\build\++ue4\sync\engine\source\runtime\core\public\misc\assertionmacros.h:161]
UE4Editor_RenderCore!FPooledRenderTarget::Release() [d:\build\++ue4\sync\engine\source\runtime\rendercore\private\rendertargetpool.cpp:1197]
UE4Editor_SlateRHIRenderer!FViewportInfo::~FViewportInfo() [d:\build\++ue4\sync\engine\source\runtime\slaterhirenderer\private\slaterhirenderer.h:126]
UE4Editor_SlateRHIRenderer!FViewportInfo::`scalar deleting destructor'()
UE4Editor_SlateRHIRenderer!FSlateRHIRenderer::OnWindowDestroyed() [d:\build\++ue4\sync\engine\source\runtime\slaterhirenderer\private\slaterhirenderer.cpp:479]
UE4Editor_Slate!FSlateApplication::PrivateDestroyWindow() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:7212]
UE4Editor_Slate!FSlateApplication::DestroyWindowsImmediately() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:3072]
UE4Editor_Slate!FSlateApplication::RequestDestroyWindow() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:2389]
UE4Editor_Slate!FSlateApplication::CloseAllWindowsImmediately() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:3025]
UE4Editor_Slate!FSlateApplication::Shutdown() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:935]
UE4Editor!FEngineLoop::Exit() [d:\build\++ue4\sync\engine\source\runtime\launch\private\launchengineloop.cpp:3431]
UE4Editor!GuardedMain() [d:\build\++ue4\sync\engine\source\runtime\launch\private\launch.cpp:179]
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:275]
UE4Editor!__scrt_common_main_seh() [d:\agent\_work\3\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
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