Struggling with Virtual DocType

Hi there,
I’m experimenting with Virtual DocType and I’m stuck just a few steps before reaching my goal.
I’m trying to populate this Virtual DocType with some data:

This is my controller:

class SchluesselVirtual(Document):
	
	def db_insert(self):
		pass

	def load_from_db(self):
		print("load_from_db")
		print(self)

	def db_update(self):
		pass

	def get_list(self, args):
		result = get_data()
		print("get_list")
		print(result)
		return result

def get_data():
	# return [{"nummer" : "123", "bezeichnung": "OneTwoThree"}, {"nummer" : "456", "bezeichnung": "FourFiveSix"}]
	return frappe.db.sql("select distinct bezeichnung, nummer from tabSchluessel", as_dict=1)

As you can see I tried to provide some data as a dictionary but then settled for a sql-query as a data source. There is the same problem with both ways.

Unfortunately the List View is not populated with the data. There is also some strange error in the console window:

These errors don’t appear when displaying normal DocTypes.

The log looks quite normal to me:

17:01:02 web.1            | 127.0.0.1 - - [02/Nov/2021 17:01:02] "GET /api/method/frappe.desk.reportview.get_sidebar_stats?stats=%5B%22_user_tags%22%5D&doctype=Schluessel&filters=%5B%5D&_=1635867960301 HTTP/1.1" 200 -
17:02:59 web.1            | load_from_db
17:02:59 web.1            | SchluesselVirtual(Schluessel Virtual)
17:02:59 web.1            | get_list
17:02:59 web.1            | [{'bezeichnung': 'Garage', 'nummer': '123'}, {'bezeichnung': 'Wohnungstür', 'nummer': 'ABC'}, {'bezeichnung': 'Haustür', 'nummer': 'XYZ'}]
17:02:59 web.1            | 127.0.0.1 - - [02/Nov/2021 17:02:59] "POST /api/method/frappe.desk.reportview.get HTTP/1.1" 200 -

I think there is something missing in the controller. Can anyone help me out?