Crash when removing cloth apb's assigned to asset

When removing already assigned cloth asset from a skeletal mesh it will crash the editor. I was able to reproduce this with the Owen asset from Content Examples, but was unable to get it to work from a newly created asset.

This behavior did not happen in 4.10.4 with the same asset, but in 4.11, 4.12, and 4.13 this happens when removing the Cloth Coat APB from the material element.

I've attached a sample project with the Owen Character in 4.11 that reproduces the crash as you remove the APB.

Test Project link: [Link Removed]

1. Open the test project
2. Open the Owen Skeletal mesh
3. Set the cloth apb assigned (Owen_Cloth_MESH_Owen_Coat.apb) to Material Element 0 to "None"

Regression: Yes

Frequency: 3/3

Results: Editor will crash

Expected: Cloth asset can be unassigned from the material element.

UE4Editor_Core!FDebug::AssertFailed() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\core\private\misc\outputdevice.cpp:430]
UE4Editor_Engine!TGPUSkinAPEXClothVertexFactoryShaderParameters::SetMesh() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\engine\private\gpuskinvertexfactory.cpp:746]
UE4Editor_Renderer!FMeshMaterialShader::SetMesh<FRHIVertexShader * __ptr64>() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\shaderbaseclasses.cpp:467]
UE4Editor_Renderer!TBasePassVertexShaderPolicyParamType<FUniformLightMapPolicyShaderParametersType>::SetMesh() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\basepassrendering.inl:16]
UE4Editor_Renderer!TBasePassDrawingPolicy<FUniformLightMapPolicy>::SetMeshRenderState() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\basepassrendering.h:909]
UE4Editor_Renderer!FDrawBasePassDynamicMeshAction::Process<FUniformLightMapPolicy>() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\basepassrendering.cpp:421]
UE4Editor_Renderer!ProcessBasePassMesh<FDrawBasePassDynamicMeshAction>() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\basepassrendering.h:1242]
UE4Editor_Renderer!FBasePassOpaqueDrawingPolicyFactory::DrawDynamicMesh() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\basepassrendering.cpp:472]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::RenderBasePassDynamicData() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\deferredshadingrenderer.cpp:426]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::RenderBasePass() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\deferredshadingrenderer.cpp:1875]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::Render() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\deferredshadingrenderer.cpp:1060]
UE4Editor_Renderer!RenderViewFamily_RenderThread() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\renderer\private\scenerendering.cpp:1744]
UE4Editor_Renderer!TGraphTask<`FRendererModule::BeginRenderingViewFamily'::`20'::EURCMacro_FDrawSceneCommand>::ExecuteTask() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\core\public\async\taskgraphinterfaces.h:886]
UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\core\private\async\taskgraph.cpp:779]
UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\core\private\async\taskgraph.cpp:526]
UE4Editor_RenderCore!RenderingThreadMain() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\rendercore\private\renderingthread.cpp:310]
UE4Editor_RenderCore!FRenderingThread::Run() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\rendercore\private\renderingthread.cpp:417]
UE4Editor_Core!FRunnableThreadWin::Run() [d:\buildfarm\buildmachine_++ue4+release-4.11\engine\source\runtime\core\private\windows\

Cloth APEX Change bug