oracle apex master details "ORA-01400" cant insert null into foreign key

 Create a new process after the master process and before the interactive grid process, make the editable region the interactive grid, include the below pl/sql code in the new process:

begin

    case :APEX$ROW_STATUS

    when 'C' then

        :QUOTATION_ID:=:P19_QUOTATION_ID;  -- The foreign key

    when 'U' then

    null;

    when 'D' then

        null;

    end case;

end;