Update dashboard.js

[Phpvarious]

Description

Lorsqu'on quitte le dashboard avant la fin du chargement complet, une erreur de type TypeError est émise :

Suggested changelog entry

Correction d’un bug de type TypeError sur le dashboard lorsqu'on quitte trop vite la page.

Types of changes

• [x] Bug fix (non-breaking change which fixes)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
  • [ ] This change is only breaking for integrators, not for external standards or end-users.
• [ ] Documentation improvement

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the [La ligne directrice pour contribuer à ce projet / Contribution guidelines for this project).
• [x] I grant the project the right to include and distribute the code under the GNU.
• [x] I have added tests to cover my changes.
• [x] I have verified that the code complies with the projects coding standards.
• [ ] [Required for new sniffs] I have added MD documentation for the sniff.