AttributeError: 'list' object has no attribute Error (detectron2)

## Instructions To Reproduce the 🐛 Bug:
1. I ran the following code in google colab to retrain the model on my custom dataset. "https://colab.research.google.com/drive/16jcaJoc6bCFAQ96jDe2HwtXj7BMD_-m5#scrollTo=PIbAM2pv-urF"

The following error "AttributeError: 'list' object has no attribute 'items'" appeared at this stage of the code:
```

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I tried to prepare and check my dataset preparation using the above code as mentioned in the google colab tutorial.
Thanks.

Start with the linked Colab notebook and the get_balloon_dicts function in the issue body; inspect the structure loaded from dataset.json and the regions value before the .items() call. Reproduce the failure with the supplied custom dataset and compare its JSON shape with the tutorial input. Done means identifying whether the mismatch is in the dataset or detectron2 and documenting the confirmed cause.

Written by the indexing model from the issue text.