Have a form in an Excel file. Have a Control on the form
("ControlX"), default value of which is "". When entering
ControlX, check is performed to ensure a prior control
contains an appropriate value. When exiting ControlX
(i.e., upon AfterUpdate), ControlX value is saved to a
global variable for later use. All this works fine during
the normal sequence of events.

However, if the user returns to ControlX and deletes the
value (as in "Oops, I made a mistake"), and then tries to
go to another control, he gets a "Invalid Property Value"
error message generated by Microsoft Forms (or so I infer
from the Blue Bar on the error message popup).

I've tried a couple of different error-trapping structures
to deal with the fact that something out there in the
background obviously doesn't like a null value in
ControlX, including one that set the value back to ""
instead of Null, but so far no luck, still get the
message. Anyone have any suggestions?