prevent picker register on background mode

OP_material_picker.py

@@ -326,11 +326,17 @@ GP_OT_pick_closest_material,
 )
 
 def register():
+    if bpy.app.background:
+        return
+
     for cls in classes:
         bpy.utils.register_class(cls)
     register_keymaps()
 
 def unregister():
+    if bpy.app.background:
+        return
+
     unregister_keymaps()
     for cls in reversed(classes):
         bpy.utils.unregister_class(cls)