Fixed crash in execution dialog

1. Connect to any database server.
2. Right click on the database(big in size) and select maintenance.
3. Start taking maintenance and click the cancel button.
4. After clicking on cancel, immediately click on cross (x) button.

With the above steps Abort() function gets called twice on from cancel event and another from close event. Attached is the patch file. Please review it and if it looks good to you then can you please commit it.