Use the correct variable in baseselector to prevent error

[stevenyoungs]

PR #1836 introduced a bug when selecting a single object.
This PR fixes it.

To reproduce:

1. using a db with at least one person
2. Shift+Alt+F to add a new family
3. Click the select child button
4. Select a child and click OK

An error similar to the following is generated

18794: ERROR: grampsapp.py: line 188: Unhandled exception
Traceback (most recent call last):
  File "C:\msys64\home\steve\gramps\gramps\gui\editors\editfamily.py", line 303, in share_button_clicked
    person = sel.run()
             ^^^^^^^^^
  File "C:\msys64\home\steve\gramps\gramps\gui\selectors\baseselector.py", line 248, in run
    result = self.get_from_handle_func()(id_list[0])
                                         ^^^^^^^
NameError: name 'id_list' is not defined