MDI form cannot be shown modally (403)

■ A form with its FormType property set to 1 - MDI
  cannot be modal.
 
■ Possible solution:
 
  • Remove or change the style% argument you have specified
    with the SHOW method.
 
  • Change the form's FormType property to 0 - Normal and
    execute the existing SHOW method again.
 
■ If you need additional help, move the cursor to a keyword
  and press F1 to open the Help window.