GPencil: Use default material when slot material is empty

When a material slot is empty, the default material is used.

The default color used is Gray to get a good contrast in dense scenes with dark and white background.

Reviewers: brecht, dfelinto, mendio, pepeland

Differential Revision: https://developer.blender.org/D5625

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -892,7 +892,8 @@ class GreasePencilMaterialsPanel:
             if is_view3d and brush is not None:
                 gp_settings = brush.gpencil_settings
                 if gp_settings.use_material_pin is False:
-                    ma = ob.material_slots[ob.active_material_index].material
+                    if ob.active_material_index > 0:
+                        ma = ob.material_slots[ob.active_material_index].material
                 else:
                     ma = gp_settings.material