Updating you Python3.7 might change the behavior.  If I read

https://github.com/python/cpython/commits/master/Lib/argparse.py

correctly, subparsers are no longer required by default (it reverted back to earlier Py3 behavior).  There is a proposed patch for handling this 'dest=None' issue, but I don't think that's been pushed.

But simply adding the 'dest' to subparses should work without update.  If you want to identify which subparser was used, you need this 'dest'.