arcgis desktop - ArcCatalog won't close/exit normally, toolbars disappear instead or it crashes

I've had this problem since v10.2 of ArcGIS Desktop, I recently upgraded to 10.3 and it still exists. The problem is that ArcCatalog cannot be closed normally. If I try to close it using the Windows close button (red X in Windows 7), instead of closing, all my toolbars disappear, including the menu toolbar. If I try to close it using File > Exit or by pressing Alt-F4, it crashes with the message:

ArcGIS for Desktop has encountered a serious application error and is unable to continue.

On the bright side, at least that closes the program. This problem doesn't affect ArcMap.

So if I use the Close button in ArcCatalog, I go from this:

To this:

The only way to close the program at that point is to kill the process in Task Manager or crash it using Alt-F4. Has anyone else had this problem and solved it? The only info I could find about it online concerned an issue in ArcGIS that was fixed in v10.1 SP1.

Answer

The problem ended up being my Xtools extension, I was using an older version and while the tools still worked with ArcGIS 10.2/10.3, it caused this problem. Updating to an ArcGIS 10.3-compatible version of the extension solved the problem.