DX12 crash when fullscreen app looses focus.

• run attached project in standalone game mode
• hit 'enter' to switch to fullscreen 1080p
• alt-tab out of the process
• process will crash

OR

• Create a new project
• Set it to run DX12
• Launch in Standalone game mode (-game or through editor)
• switch to 1080p from the console (r.setres 1920x1080f)
• alt-tab out of the process
• process will crash

UE4Editor_D3D12RHI!D3D12RHI::VerifyD3D12Result() [d:\build\++ue4\sync\engine\source\runtime\d3d12rhi\private\d3d12util.cpp:263]
UE4Editor_D3D12RHI!FD3D12Viewport::PresentChecked() [d:\build\++ue4\sync\engine\source\runtime\d3d12rhi\private\d3d12viewport.cpp:553]
UE4Editor_D3D12RHI!FD3D12Viewport::Present() [d:\build\++ue4\sync\engine\source\runtime\d3d12rhi\private\d3d12viewport.cpp:741]
UE4Editor_D3D12RHI!FD3D12CommandContextBase::RHIEndDrawingViewport() [d:\build\++ue4\sync\engine\source\runtime\d3d12rhi\private\d3d12viewport.cpp:886]
UE4Editor_RHI!FRHICommand<FRHICommandEndDrawingViewport,FRHICommandEndDrawingViewportString1859>::ExecuteAndDestruct() [d:\build\++ue4\sync\engine\source\runtime\rhi\public\rhicommandlist.h:726]
UE4Editor_RHI!FRHICommandListExecutor::ExecuteInner_DoExecute() [d:\build\++ue4\sync\engine\source\runtime\rhi\private\rhicommandlist.cpp:351]
UE4Editor_RHI!FExecuteRHIThreadTask::DoTask() [d:\build\++ue4\sync\engine\source\runtime\rhi\private\rhicommandlist.cpp:410]
UE4Editor_RHI!TGraphTask<FExecuteRHIThreadTask>::ExecuteTask() [d:\build\++ue4\sync\engine\source\runtime\core\public\async\taskgraphinterfaces.h:847]
UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() [d:\build\++ue4\sync\engine\source\runtime\core\private\async\taskgraph.cpp:686]
UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() [d:\build\++ue4\sync\engine\source\runtime\core\private\async\taskgraph.cpp:583]
UE4Editor_RenderCore!FRHIThread::Run() [d:\build\++ue4\sync\engine\source\runtime\rendercore\private\renderingthread.cpp:289]
UE4Editor_Core!FRunnableThreadWin::Run() [d:\build\++ue4\sync\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:96]