Fix #132117: Grease Pencil: Material slots display logic fix

Sometimes context.material.grease_pencil will be none. E.g. when adding
a material slot in some grease pencil objects. This fix modified the
`poll` function to show material slots when the active object is a
grease pencil object or an active grease pencil material is present in
the context.

Pull Request: https://projects.blender.org/blender/blender/pulls/132158

scripts/startup/bl_ui/properties_material_gpencil.py

@@ -92,8 +92,9 @@ class MATERIAL_PT_gpencil_slots(GreasePencilMaterialsPanel, Panel):
     @classmethod
     def poll(cls, context):
         ma = context.material
-
-        return ma and ma.grease_pencil
+        found_material = ma and ma.grease_pencil
+        found_object = context.object and context.object.type == 'GREASEPENCIL'
+        return found_material or found_object
 
 
 # Used as parent for "Stroke" and "Fill" panels