Like it says…crashrep at the link:
I can confirm this happens to me too, on macOS Sierra 10.12.4. Program worked fine before I updated the OS to the latest version.
Right now the program crashes, and then on subsequent re-launches it offers to either try to reopen windows or don’t reopen them; no matter whether you choose “don’t reopen” or “reopen” it then crashes with the same stacktrace.
I also tried uninstalling and reinstalling the program, but the freshly-installed program still crashed with the same message.
Any ideas for a workaround in the meantime?
The updated MacOS was incompatible with that version of Shotcut. Fortunately the new release 17.04 is fixed and works again with the OS updates. Get it here 
I can confirm that release 17.04 works with macOS 10.12.5. Thanks for the quick fix!