Thanks mangakissa, FTurtle
With further tracing of the error, I found that it occurred at line 2883 in customform.inc... Code below.
Line 2883 is executed if the forms PopupMode = pmNone.
I changed the forms PopupMode to pmAuto in the IDE via Object Inspector, and the application worked OK.
Confirmed by changing the property back to pmNone, and the application created the error again.
customform.inc
...
2804 function TCustomForm.ShowModal: Integer;
...
...
2877 Application.ModalStarted;
2878 try
2879 Include(FFormState, fsModal);
2880 if PopupMode = pmNone then <== <==
2881 begin
2882 RecreateWnd(Self);
2883 HandleNeeded;
2884 end;