Hey there @jorny! I can’t seem to replicate the issue. Checking the source code for the function the error stems from hasn’t been fruitful, since it’s a utility function int32 FGeomObject::GetObjectIndex(). This is basically used to get the index of the geometry object that’s selected. The return check(0); // Should never happen is never meant to fire, it’s basically the failsafe in case there is no way to get the object’s index appropriately.
This makes it difficult to debug without attaching a debugger on your side to see what called it and what data it had when it was called. Does this occur when working with any Brushes whatsoever?
Link to the source file referenced (this link requires you to have tied your Epic account to Github to access the engine’s source code, otherwise the page will 404): https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Editor/GeometryMode/Source/GeometryMode/Private/EditorGeometry.cpp