I'm having trouble replicating it, so if there's anything you can do to guarantee that it'll happen I'd love to know what you do.
...OK for example:
1. Purchased a Ruger Warhawk, went to add Personalised Grip (which appears twice in the listing) and when I click on OK, the Unhandled Exception error pops up. It occurred with any other mod like Trigger Removal, Quick Draw Holster, etc.
2. Chose an Ares Predator V and the same issue occurred.
3 when I clicked "Delete" for another weapon I was using as a test, the error popped up again.
It seems that some accessories work fine while others don't as I was able to add a Bayonet to an M1 Garand, but not "Custom Look" or a Shock Pad...
This didn't occur in ver .175 (the last update I used).
Also I noticed that in viewing the .xml file for weapons in the .177 Data file in notepad, all the code looks "jumbled" and is not formatted line by line like in .ver 175.