[AccessD] Check Subform for NotNull

I have a form and a subform where the users want the subform locked if a
record has been entered. They do not want it to be changed once data has
been entered. They can add new records, just not edit existing records.

 

Then I have the problem - 'what if they make a typo' and discover it
after the record has been entered and locked. They only want a
supervisor to be able to go back, unlock the record on the subform and
make the change.

 

I tried the below code OnOpen of the Main form, but receive an error.

 

    If
IsNotNull(Forms!frm_InventoryMain.frm_InventorySubform.InventoryID) Then

        Forms!frm_InventoryMain.frm.InventorySubform.Form = Locked

    Else    

 

    End If

 

Virginia