■ In some situations, you are not allowed to unload a form or
  control on a form. This error may be caused by trying to
  unload a form or control on the form:
 
  • During any Paint event for the form or any control on the
    form.
 
  • Whenever a combo box control is opened either on the form
    being unloaded or on the form that contains the control
    being unloaded.
 
■ Possible solution:
 
  • Remove the UNLOAD statement from the Paint event procedure
    or disable the combo box during the Unload event.
 
■ If you need additional help, move the cursor to a keyword
  and press F1 to open the Help window.