NameError when doing rasa_core training with custom actions_action.py

rasa_nlu version: 0.13.3 rasa_core version: 0.13.0a1

I got the following error when I do rasa_core training with custom actions_action.py file.

NameError: Can not access action '', as that name is not a registered action for this domain. Available actions are:
         - action_listen
         - action_restart
         - action_default_fallback
         - action_deactivate_form
         - utter_bmgs
         - utter_greet
         - utter_solution_hkisland
         - utter_bmgs_single
         - utter_bmgs_owner
         - utter_solution_nt
         - utter_transfer
         - utter_goodbye
         - utter_unqualified
         - utter_congrats_form
         - utter_wealth_low
         - utter_defective_windows_solution
         - utter_wealth_high
         - utter_unclear
         - utter_bmgs_both
         - utter_water_leakage
         - utter_solution_kowloon
         - utter_thanks
         - action_search_form
         - action_congrats_form

But I found that all the above names are included in my domain_action.yml file. Does anyone encounter the same problem?