I ran into an issue with iReport 4.5.0 and 4.5.1 where clicking the database icon to edit the query would cause the (modal) query editing dialog to not display which required a force close of the application.
I am using a Windows 7 machine with two monitors of different resolution, which I am expecting to be part of the problem. The issue is with the line:
in the file ~/AppData/Roaming/.ireportpro/4.5.1/config/Preferences/com/jaspersoft/ireport.properties
Obviously, a negative x coordinate would render the dialog outside of my resolution, so resetting that value to 0 solves the issue.
I didn't see anything on this site when I searched for the issue and figured it out myself and wanted to share.