Jumping Between Forms - Making A Form Visible And Not Visible

I'm having an odd problem when making a form visible and not visible and it not continuing to function. Here is what I have happening. I have a main form (frmMain) that is an MDIParent. On this form I have a menu to open a second form (frmCalculate) and populate a listview from Items in the database.

As I click or select items in the listview on frmCalculate the tag is read and data is pulled based on the id stored in that tag and fills in various fields on the form. This works great. However, I also have a context menu attached to the listview that allows me to perform a "what if" scenerio on the items I am calculating. When I select this menu, I hide the frmCalculate and open the frmWhatIf form where I can mess around with values on the item I had hilighted in the listview on frmCalculate. Nothing is pulled from or written back to the database here, all the information is filled in from fields on frmCalculate. It just allows me to look at rising costs and how they will affect my margins.... anyway.

If I then close frmWhatIf, it brings my frmCalculate back by setting its visible propery to true (never closed frmCalculate, just hid it). At this point if I click an item in the listview I get an exception for a null reference. IF however I never hide frmCalculate, I can open and close frmWhatIF without ever having an issue. Why do I lose the functionality to select items just by hiding and unhiding the form? As a test I added two menus to the toolbar on frmMain one called hide and one called show. I then opened frmCalculate with original menu item to populate the listview and selected a few items to test that it was working and then using the hide menu I made the form invisible and the show button to bring it back. This yielded the same exception so without even opening frmWhatIf the problem still occurs.