If I was trying to do this, this is how I would get to the same place you
want to be. Make the form popup or maximised and modal, this will stop the
user from clicking anywhere else to move focus from the current form. Turn
off all the forms close controls in the forms properties. Now you have a form
you can open but not close.
To close the form. Add a close button. Use the wizard to generate a button
to close the current form (DoCmd.Close (lookup the syntax in help for your
access version))
In the forms on open event, disable the close button (Me.ButtonName.Enabled
= False)
In the Control that is not to be Null, After Update event re-enable the
button (Me.ButtonName.Enabled = true) (You may have to play with which event
of the Control carries the Enable code)
I hope I have explained carefully and that this solution helps you.
Kindest Regards
Mike
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Kindest Regards Mike B