Error using generate_list.py

It returns the following message:
```
Traceback (most recent call last):
  File "generate_list.py", line 24, in <module>
    main()
  File "generate_list.py", line 15, in main
    xml_files = [f for f in os.listdir(results_dir) if
OSError: [Errno 2] No such file or directory: ''
```