TypeError: object of type ‘NoneType‘ has no len()

After executing the following command python tools/run_net.py --cfg demo/AVA/SLOWFAST_32x2_R101_50_50.yaml, you will obtain the following results:
Traceback (most recent call last):
File "tools/run_net.py", line 42, in
main()
File "tools/run_net.py", line 38, in main
demo(cfg)
File "/home/qro/qrl/SlowFast/tools/demo_net.py", line 115, in demo
for task in tqdm.tqdm(run_demo(cfg, frame_provider)):
File "/home/qro/miniconda3/envs/slowfast/lib/python3.8/site-packages/tqdm/std.py", line 1181, in __iter__
for obj in iterable:
**File "/home/qro/qrl/SlowFast/tools/demo_net.py", line 42, in run_demo
if len(cfg.DEMO.LABEL_FILE_PATH) != 0
TypeError: object of type 'NoneType' has no len()**