BUG: Mass Renaming substring can crash cam2pc

If you have long filenames and use the mass renaming on them to extract a substring from the end, e.g. substr:20, when you next use the mass rename feature on a file with a shorter name (e.g. less than 20 with the above example), cam2pc will crash. I assume this is because it is trying to show a preview of the first selected filename, and substring:20 is beyond the end of the name.