Using Custom Primitive Data as Final Colour in a UI Material crashes the engine with a missing uniform buffer

Using custom primitive data as an input in a UI material causes the engine to crash in the RHI thread. The exact crash is "Missing uniform buffer at slot 1, stage SF_Pixel." at Engine\Source\Runtime\D3D12RHI\Private\D3D12DescriptorCache.cpp:676.

Reproduced on 5.4 via EGL and from Git on commit 7cbf0db9e39bd2cf7ed4fad0bc67e4f9b61611ae. Both tests under D3D12 SM6

1. Create a material
2. Create a parameter and select "Use Custom Primitive Data"
3. Change to UI shader type
4. Connect parameter to the final colour
5. Engine will crash in the RHI thread.