Variable Window Pop-up not showing

I created a simple SQL statement that takes 2 input variables. When I run the statement, hitting F8, the popup window does not show and it appears to be in background?? how can I get this window to show? I cannot TAB to the cancel button.
 
Can you start regedit.exe and delete the following section?

HKEY_CURRENT_USER\Software\Allround Automations\PL/SQL Developer\Position\SubstVarForm
 
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