Use Viewport's BackBuffer as Pixel Shader's input texture fails on Android

UDK Link: link title
In this project, they need to get the BackBuffer(including UMG), and use this BackBuffer to do some post-processing. So follow the code in FViewportSurfaceReader :: ResolveRenderTarget, to get BackBuffer after Slate Windows Rendered.
The error code is in fucntion FScreenCaptureModule::OnSlateWindowRendered_ScreenCapture, the main code is:

FTexture2DRHIRef SourceBackBuffer = RHICmdList.GetViewportBackBuffer(*ViewportRHI);
PixelShader->SetParameters(RHICmdList, ..., SourceBackBuffer);
RendererModule->DrawRectangle(...);

This is successful on a PC, but fails on Android.

[Step 1], unzip "AndroidSceneCapture.zip", generate VS project.
[Step 2], start project, open map "/Content/ScreenCapture/Test/TestLevel".
[Step 3], Play, and click button in screen, screen capture is successful.
[Link Removed]
[Step 4], Package into a Android device, and click button in device, screen capture is failed.