mmdetection: TypeError: 'NoneType' object is not subscriptable

Describe the bug While training the model I am facing this error, not sure where am I going wrong.

Reproduction

1. What command or script did you run?

!python tools/train.py  'configs/truck/cascade_mask_rcnn_swin_small_patch4_window7_mstrain_480-800_giou_4conv1f_adamw_3x_truck.py' 

Using the pre-trained model as a checkpoint for transfer learning.

1. Did you make any modifications on the code or config? Did you understand what you have modified? Yes, I modified the code for custom object detection for the custom dataset. Using a custom dataset with modifications made in the config/swin/xxx file and changed the path in the _ base _ variable in the config file downloaded from the repository.

2. What dataset did you use? custom dataset with a json file annotations

Environment

1. Please run python mmdet/utils/collect_env.py to collect necessary environment information and paste it here. I don’t think it is env problem, I have no issues running a pre-trained model by itself.

2. You may add addition that may be helpful for locating the problem, such as

   • How you installed PyTorch [e.g., pip, conda, source]
   • Other environment variables that may be related (such as $PATH, $LD_LIBRARY_PATH, $PYTHONPATH, etc.)

I have modified the files that were mentioned in the config files like num_classes and adding data variables with paths, enabling optimizer_config and in models/cascade_xxx.py.

Error traceback

File "tools/train.py", line 188, in <module>
    main()
  File "tools/train.py", line 177, in main
    train_detector(
  File "/home/w/cbn/CBNetV2/mmdet/apis/train.py", line 185, in train_detector
    runner.run(data_loaders, cfg.workflow)
  File "/home/w/cbn/lib/python3.8/site-packages/mmcv/runner/epoch_based_runner.py", line 127, in run
    epoch_runner(data_loaders[i], **kwargs)
  File "/home/w/cbn/lib/python3.8/site-packages/mmcv/runner/epoch_based_runner.py", line 47, in train
    for i, data_batch in enumerate(self.data_loader):
  File "/home/w/cbn/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 521, in __next__
    data = self._next_data()
  File "/home/w/cbn/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 1203, in _next_data
    return self._process_data(data)
  File "/home/w/cbn/lib/python3.8/site-packages/torch/utils/data/dataloader.py", line 1229, in _process_data
    data.reraise()
  File "/home/w/cbn/lib/python3.8/site-packages/torch/_utils.py", line 425, in reraise
    raise self.exc_type(msg)
TypeError: Caught TypeError in DataLoader worker process 0.
Original Traceback (most recent call last):
  File "/home/w/cbn/lib/python3.8/site-packages/torch/utils/data/_utils/worker.py", line 287, in _worker_loop
    data = fetcher.fetch(index)
  File "/home/w/cbn/lib/python3.8/site-packages/torch/utils/data/_utils/fetch.py", line 44, in fetch
    data = [self.dataset[idx] for idx in possibly_batched_index]
  File "/home/w/cbn/lib/python3.8/site-packages/torch/utils/data/_utils/fetch.py", line 44, in <listcomp>
    data = [self.dataset[idx] for idx in possibly_batched_index]
  File "/home/w/cbn/CBNetV2/mmdet/datasets/custom.py", line 194, in __getitem__
    data = self.prepare_train_img(idx)
  File "/home/w/cbn/CBNetV2/mmdet/datasets/custom.py", line 217, in prepare_train_img
    return self.pipeline(results)
  File "/home/w/cbn/CBNetV2/mmdet/datasets/pipelines/compose.py", line 40, in __call__
    data = t(data)
  File "/home/w/cbn/CBNetV2/mmdet/datasets/pipelines/loading.py", line 371, in __call__
    results = self._load_masks(results)
  File "/home/w/cbn/CBNetV2/mmdet/datasets/pipelines/loading.py", line 323, in _load_masks
    [self._poly2mask(mask, h, w) for mask in gt_masks], h, w)
  File "/home/w/cbn/CBNetV2/mmdet/datasets/pipelines/loading.py", line 323, in <listcomp>
    [self._poly2mask(mask, h, w) for mask in gt_masks], h, w)
  File "/home/w/cbn/CBNetV2/mmdet/datasets/pipelines/loading.py", line 281, in _poly2mask
    elif isinstance(mask_ann['counts'], list):
TypeError: 'NoneType' object is not subscriptable