Script exit with no output

---

Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "./src/main.py", line 55, in <module>
    run()
  File "./src/main.py", line 44, in run
    tasks_functions[task](source_dir, output_dir, print_to_stdout)
  File "./src/task4.py", line 214, in run_threads_task
    result, _ = run_clustering(html_generator, output_dir, sort_threads=False, print_plots=print_plots)            
  File "./src/task4.py", line 99, in run_clustering
    tokens = set([t for t in WORD_RE.findall(cleaned_t) if t not in LANG_STOP_WORDS[lang]])
  File "./src/task4.py", line 99, in <listcomp>
    tokens = set([t for t in WORD_RE.findall(cleaned_t) if t not in LANG_STOP_WORDS[lang]])
KeyError: None