This happens when Rocksmith can't be found through Steam library folders, but should hopefully be fixed in v0.0.4. If not could you open an issue at https://github.com/andulv/RSTabExplorer/issues
Version 0.0.4 released. https://github.com/andulv/RSTabExplorer/releases Fixes some incorrect chords and a couple of loading related crashes. What exactly happens? Does 0.0.4 work? Are you able to open songs.psarc from your Rocksmith folder using the "Load: Psarc File" button?
I've added error handling for this so it won't crash so violently. It will be in v0.0.4, but I'm waiting to get feedback on changes I made to fix incorrect chords before I compile/release it. I plan to get printing working at some stage. Anders (Creator of RS Tab Explorer) also has tab export planned but is looking for help because he is doesn't have much time to work on this project at the moment.