Gantt warning 'Diagram is readonly' appears twice

[abrassat]

Describe the bug
A clear and concise description of what the bug is.

Steps to reproduce
Steps to reproduce the problematic behavior:

1. Insert a gantt macro in a page (readonly=true)
2. Try to modify a task (start/end date, progress)

Expected behavior
The warning appears once

Actual behavior
The warning appears twice

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: linux mint
• Browser firefox
• XWiki version 15.10
• App version 3.11.0

Additional context
Add any other context about the problem here.