Shader Editor: tweak when the UI for managing the material is enabled

- disable the UI if the currently selected object does not support materials.
- add Metaballs and GPencil as types supporting material.

release/scripts/startup/bl_ui/space_node.py

@@ -63,10 +63,12 @@ class NODE_HT_header(Header):
                 layout.separator_spacer()
 
                 row = layout.row()
+                types_that_support_material = {'MESH', 'CURVE', 'SURFACE', 'FONT', 'META', 'GPENCIL'}
                 # disable material slot buttons when pinned, cannot find correct slot within id_from (#36589)
-                row.enabled = not snode.pin
+                # disable also when the selected object does not support materials
+                row.enabled = not snode.pin and ob.type in types_that_support_material
                 # Show material.new when no active ID/slot exists
-                if not id_from and ob.type in {'MESH', 'CURVE', 'SURFACE', 'FONT', 'METABALL'}:
+                if not id_from and ob.type in types_that_support_material:
                     row.template_ID(ob, "active_material", new="material.new")
                 # Material ID, but not for Lights
                 if id_from and ob.type != 'LIGHT':