Fix AttributeError in SubmissionCreationSerializer when data is None

[Didayolo]

Description

SubmissionCreationSerializer.get_filename() (line 121) crashes with AttributeError: 'NoneType' object has no attribute 'data_file' when submission.data is None.

The same file already handles this case correctly in SubmissionListSerializer (line 69-73):

def get_filename(self, instance):
    if instance.data and instance.data.data_file:
        return basename(instance.data.data_file.name)
    return "Deleted File"

But SubmissionCreationSerializer does not:

def get_filename(self, instance):
    return basename(instance.data.data_file.name)  # crashes if data is None

Impact: A single incomplete API submission causes a 500 Internal Server Error on the competition detail page for all users, not just the caller.

Original PR

• Fix AttributeError in SubmissionCreationSerializer when data is None #2396

Reproduction steps

The REST API submission flow uses pre-signed S3 URLs in 4 steps:

# Step 1 - Create dataset record (returns pre-signed S3 URL)
curl -X POST \
  -H "Authorization: Token $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"type": "submission", "name": "My Run", "file_name": "predictions.zip", "file_size": 1234, "request_sassy_file_name": "predictions.zip"}' \
  https://<host>/api/datasets/
# Response: {"key": "<uuid>", "sassy_url": "https://s3..."}

# Step 2 - Upload file to S3
curl -X PUT -H "Content-Type: application/zip" --data-binary @predictions.zip "<sassy_url>"

# Step 3 - Mark upload complete
curl -X PUT -H "Authorization: Token $TOKEN" https://<host>/api/datasets/completed/<key>/

# Step 4 - Create submission
curl -X POST \
  -H "Authorization: Token $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"phase": 3, "data": "<key>"}' \
  https://<host>/api/submissions/

To trigger the bug: Skip steps 2-3 (S3 upload fails or is never completed) and go directly to step 4. The API happily creates a Submission record with data_id=None. Then visit /competitions/<id>/ - the page returns 500 for every user.

The traceback from Django logs:

File "/app/src/apps/api/serializers/submissions.py", line 121, in get_filename
    return basename(instance.data.data_file.name)
AttributeError: 'NoneType' object has no attribute 'data_file'

Fix

Apply the same null-check pattern from SubmissionListSerializer:

def get_filename(self, instance):
    if instance.data and instance.data.data_file:
        return basename(instance.data.data_file.name)
    return None

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge