I tried to add multiple pieces into the piece pallet (done this many times with previous versions with no issues).
Once i picked a prototype file and selected from the directory and then go. It gets a crash.
I experimented some, and if I do NOT include a prototype, then the import works fine. If any prototype is specified then it crash and burn.
Thanks for the report, will get this fixed for 3.4.9. There where 3 bug fixes in 3.4.8 for the Mass Piece Loader, but one of them has introduced this bug.