Fix T77082: Greasepencil error accessing material dropdown on an object without materials

Differential Revision: https://developer.blender.org/D7841

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -639,7 +639,7 @@ class GreasePencilMaterialsPanel:
             if is_view3d and brush is not None:
                 gp_settings = brush.gpencil_settings
                 if gp_settings.use_material_pin is False:
-                    if ob.active_material_index >= 0:
+                    if len(ob.material_slots) > 0 and ob.active_material_index >= 0:
                         ma = ob.material_slots[ob.active_material_index].material
                 else:
                     ma = gp_settings.material