This is a popular question. Like most other Inquirers, you do the same mistake: there is no parent-child relationship between forms, there is Owner and
OwnedForm
. You need to use this relationship if you want good UI integrity. For form collaboration it does not matter at all though.
As usual, the most robust approach based on the interface implemented by the form is not mentioned so far.
Please see my past answer and discussion here:
How to copy all the items between listboxes in two forms[
^].
—SA