Forward Shading: Scene Capture 2D crash when assigning a RGB Capture Source

When using Forward Shading for the project, if you assign a source target to a SceneCapture2D and use either BaseColor in RGB or Normal in RGB the editor will crash.

Tested with:
Broken: 4.14.1 Binary CL-3224400
Broken: 4.15. Dev-Rendering CL-3231252

1. Enable Forward Shading via the Project Settings.
2. In the Content Browser, create a Texture Render Target
3. Add a Scene Capture 2D to the scene
4. Set the Capture Source to "BaseColor in RGB" or "Normal in RGB"
5. Assign the created Texture RenderTarget to the Texture Target for the SceneCapture2D

Results: The editor will crash when the render target is assigned if using BaseColor in RGB or Normal in RGB.

LoginId:7d8a1a304be9658328c158a3d14cc774
EpicAccountId:7c6293d6bf1940c0bc89e1494c5ba9d7

Assertion failed: IsValidRef(GBufferResourcesUniformBuffer) [Link Removed] [Line: 426]
GBuffer required but not available. Failure often caused by incorrect custom code use in a post processing material.

UE4Editor_Core!FDebug::AssertFailed() [d:\builds\releases\engine\source\runtime\core\private\misc\assertionmacros.cpp:349]
UE4Editor_Renderer!FSceneRenderTargets::GetGBufferResourcesUniformBuffer() [d:\builds\releases\engine\source\runtime\renderer\private\postprocess\scenerendertargets.h:428]
UE4Editor_Renderer!FDeferredPixelShaderParameters::Set<FRHIPixelShader * __ptr64,FRHICommandList>() [d:\builds\releases\engine\source\runtime\renderer\private\postprocess\scenerendertargets.cpp:2527]
UE4Editor_Renderer!TSceneCapturePS<6>::SetParameters() [d:\builds\releases\engine\source\runtime\renderer\private\scenecapturerendering.cpp:84]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::CopySceneCaptureComponentToTarget() [d:\builds\releases\engine\source\runtime\renderer\private\scenecapturerendering.cpp:191]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::Render() [d:\builds\releases\engine\source\runtime\renderer\private\deferredshadingrenderer.cpp:1248]
UE4Editor_Renderer!UpdateSceneCaptureContentDeferred_RenderThread() [d:\builds\releases\engine\source\runtime\renderer\private\scenecapturerendering.cpp:244]
UE4Editor_Renderer!UpdateSceneCaptureContent_RenderThread() [d:\builds\releases\engine\source\runtime\renderer\private\scenecapturerendering.cpp:285]
UE4Editor_Renderer!`FScene::UpdateSceneCaptureContents'::`12'::EURCMacro_CaptureCommand::DoTask() [d:\builds\releases\engine\source\runtime\renderer\private\scenecapturerendering.cpp:566]
UE4Editor_Renderer!TGraphTask<`FScene::UpdateSceneCaptureContents'::`12'::EURCMacro_CaptureCommand>::ExecuteTask() [d:\builds\releases\engine\source\runtime\core\public\async\taskgraphinterfaces.h:883]
UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() [d:\builds\releases\engine\source\runtime\core\private\async\taskgraph.cpp:954]
UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() [d:\builds\releases\engine\source\runtime\core\private\async\taskgraph.cpp:701]
UE4Editor_RenderCore!RenderingThreadMain() [d:\builds\releases\engine\source\runtime\rendercore\private\renderingthread.cpp:325]
UE4Editor_RenderCore!FRenderingThread::Run() [d:\builds\releases\engine\source\runtime\rendercore\private\renderingthread.cpp:459]
UE4Editor_Core!FRunnableThreadWin::Run() [d:\builds\releases\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:76]