Cannot specify branch for pull request trigger and cannot draw it without specifying activity types

[AlexisNeveu]

Describe the bug

Impossible to draw a pull request trigger on a specific branch and without specifying the list of activity types.

To Reproduce

Steps to reproduce the behavior:

1. Go to text view
2. Add a pull_request field in "on" field
3. Specify the branches with "branches: [master, dev]"
4. Switch to draw view
5. See that it prevent the model to draw itself entirely

Expected behavior

If the activity types are not specified then display a warning in the draw view but draw all the model entirely.
The branches specified in the text view should appear.

Screenshots

When branches are specified and/or types are not :

When branches are not specified but types are:

Application (please complete the following information):

• leto-modelizer commit hash 280a6e5ffd4bcb75bc8e3c8d4e20c787ea067222
• githubator-plugin commit hash b02edb8

Desktop (please complete the following information):

• OS: Ubuntu
• Browser FireFox