When Blender is built using `WITH_EXPERIMENTAL_FEATURES=OFF`, the GP custom properties panel still tries to access `bpy.types.GreasePencilv3`, which then doesn't exist. This is a very simple, local fix that checks whether that type exists, and only then defines `DATA_PT_grease_pencil_custom_props`. Pull Request: https://projects.blender.org/blender/blender/pulls/121394