Your issue may be that subforms are not loaded, therefore they are not part of the forms collection. You can, however, test for a property such as Name.
Try this: open the form and try to retrieve the form Name property (or the listbox if you prefer) in the immediate window. You should get a valid result if your syntax is correct.
Then close the form, go back to the immediate window and at the end of the same line you used, hit return again. You should get a message that the object cannot be found.
So perhaps alter your code to get a property (e.g. Name) and deal with the result. It's not clear to me if there could be a different form open or just none so how to code it depends on that.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.