Client Ensure FShaderMapResource_SharedCode::ReleaseRHI is still referenced on exit/shutdown

Very rarely, the game client will crash on shutdown and the log will contain messages that look like:

LogOutputDevice: Error: Ensure condition failed: false  [File:Engine\Source\Runtime\RenderCore\Private\ShaderCodeLibrary.cpp] [Line: 1271] 
LogOutputDevice: Error: FShaderMapResource_SharedCode::ReleaseRHI is still referenced (Num of references 1, owner <material instance used in Niagara>)

The owner changes, but in the 2 cases that listed owners, both belonged to Material Instances that were used only inside Niagara Systems.

This issue occurs inconsistently on game client exit/shutdown.