How to read Apex 21.2 CheckboxGroup values in PL/SQL

I'm using Apex 21.2 on a 21.C database. I can't read the values that the user has checked when I execute a PL/SQL dynamic action.
The Checkbox Group field is called P302_MINISTRY_GROUP.
The Type = Checkbox Group. The LOV SQL Query = Select ministry || ' - ' || role display, prim_key return from vol_ministry order by ministry, role; I did not put anything into the Source, Default, or Server Side Conditions. I created a dynamic action that fires when the user presses the "Save" button.
For the TRUE Action = Execute Server-side Code. The PL/SQL code is:

declare
  l_dummy number;
  l_volunteer_pk number;
begin
  l_volunteer_pk := :P302_PRIM_KEY;
  for cur_r in (select * From table(apex_string.split(:P302_MINISTRY_GROUP, ':'))) 
  loop
    select prim_key
      into l_dummy
      from vol_contact_ministry where ministry_fkey = cur_r.column_value
                                  and contact_fkey = l_volunteer_pk;
    if l_dummy is null then
       -- checked value does not exist
       insert into vol_contact_ministry (ministry_fkey, contact_fkey, status) 
                                 values (cur_r.column_value, l_volunteer_pk, 'Active');
    --else
       -- checked value exists
    --   delete from vol_contact_ministry where ministry_fkey = cur_r.column_value
    --                                      and contact_fkey = l_volunteer_pk;
    end if;
  end loop;
end;

The checkboxes display on the screen with the concatenated fields just fine. When the user checks one or more boxes and presses "Save", I would expect that the PL/SQL would loop through the checked boxes and give the prim_key associated with that box. But I can't seem to get any values at all. As a test I tried inserting the entire field :P302_MINISTRY_GROUP but the table shows Null for that insert. The PL/SQL is firing properly when the button is pressed based on other tests that I made, but I can't get this code above to work. How do I obtain the values of checked boxes from a Checkbox Group so that I can manipulate it in PL/SQL ?