Database indexing and database view are not working anymore

When running the beat command line utility to perform one of the following operations (indexing or viewing database content), the script crashes with such an error:

./bin/beat --prefix=work db view atnt/1/idiap/train
Traceback (most recent call last):
  File "./bin/beat", line 34, in <module>
    sys.exit(beat.cmdline.scripts.beat.main())
  File "/remote/idiap.svm/user.active/tpereira/BEAT/beat.core/src/beat.cmdline/beat/cmdline/scripts/beat.py", line 160, in main
    return module.process(args)
  File "/remote/idiap.svm/user.active/tpereira/BEAT/beat.core/src/beat.cmdline/beat/cmdline/databases.py", line 400, in process
    return view_output(args['config'], args['<set_name>'])
  File "/remote/idiap.svm/user.active/tpereira/BEAT/beat.core/src/beat.cmdline/beat/cmdline/databases.py", line 330, in view_output
    input_group = view.input_group(exclude_outputs=excluded_outputs)
  File "/remote/idiap.svm/user.active/tpereira/BEAT/beat.core/beat/core/database.py", line 146, in input_group
    for output in self.outputs:
TypeError: 'NoneType' object is not iterable