I think there is something wrong with Rhino.Render.RenderMaterialTable. As far as I understand I should be able to access a specific render material in the document using scriptcontext.doc.RenderMaterials[index]. Tying to do this inside a GhPython script crashes Rhino 6 SR24. However the using the enumerator scriptcontext.doc.RenderMaterials.GetEnumerator()[index] works.
So maybe RenderMaterialTable.getitem is not implemented properly or list access is not supposed to work?