Sometimes repo = null

[n0str]

1. https://github.com/codex-bot/github/blob/master/github/data_types/branch.py
2. https://github.com/codex-bot/github/blob/master/github/data_types/repository.py

# Who merged and sends
        self.repo = None
        if 'repo' in data:
            self.repo = Repository(data['repo'])

In the webhook data payload.head.user.repo == null
so, data['repo'] == None and we initiate Repository(None)