TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType' What is the problem?

Question

def load_file():
  # Проходим циклом и загружаем все файлы из папок
  cat_files = []
  dog_files = []

  for i in (cat_name):
    df = cv2.VideoCapture(file_name_cat + i