Errors after creating and when trying to save diagrams on mobile

[AndreeaChi]

Steps to reproduce on an Android 11 phone:

1. Open a new tab on Chrome browser 96.0.46 and login on the XWiki instance
2. Go to the Diagrams main page and create a new diagram.

Result:

I clicked on the Ok button in order to get access to the diagram.

3. I have tapped on the x icon from the shown message and added a figurine from the menu. Closed the figurines menu.
   
   I noticed that the buttons are not in full saturation.
4. Tapped on Enregistrer & Fermer. It mentions the options to cancel or leave the page. In the background on the bottom I see the rolling icon on the "Enregistrement..." grey rectangle.
   
5. I tap on Leave and the result is an empty diagram page:
   

A second test:

1. Open a new tab on Firefox mobile 94.1.2 and login on the XWiki instance
2. Go to the Diagrams main page and create a new diagram.
   Result:
   
   I clicked on the Ok button in order to get access to the diagram.
3. I have tapped on the x icon from the shown message and added a figurine from the menu. Closed the figurines menu.
   
   Buttons are fully saturated.
4. Tapped on Enregistrer & Fermer. It mentions the options to cancel or leave the page. In the background on the bottom I see the Enregistre green rectangle.
   
5. I tap on Leave and the result is an empty diagram page:
   

On both pages, when I reopen them again on the same browsers on mobile, the figurines that I added before are not shown.