Can't run Video Object Tracking SAM-2 pre-annotation

**Describe the bug**
Using the video object tracking feature with sam-2 as described here: https://docs.humansignal.com/tutorials/segment_anything_2_video

**To Reproduce**
Steps to reproduce the behavior:
1. Clone and run label studio and label studio ML backend.
2. Run the [segment_anything_2_video](https://github.com/HumanSignal/label-studio-ml-backend/tree/master/label_studio_ml/examples/segment_anything_2_video) example from label studio ML backend
3. Follow all the steps in the [tutorial](https://docs.humansignal.com/tutorials/segment_anything_2_video).
4. See error in ml backend:
```
path/to/model.py in get_prompts():
for ctx in context['result']
TypeError: NoneType object is not subscriptable
```
**Expected behavior**
Predictions get computed and appear on the interface as shown in the tutorial

**Environment (please complete the following information):**
- OS: [e.g. iOS] label studio on windows 11, label studio ml backend on Ubuntu 22.04
- Label Studio Version [e.g. 0.8.0]: Cloned at commit 6da49c156863bc8f302b829674f63c1c89b873c7

**Additional context**
Looks like interactive annotation is not sending any requests. The errors seem to have been caused by pre-labelling requests.
On disabling pre-labelling, and following the interactive annotation process as shown in the tutorial GIF:
1. Toggle auto-annotation in labelling interface
2. Select label
3. Draw bounding box

No requests seem to be sent to the ML backend.