You must have done more than just that because:
- Diagram Application version 1.2 (the free version) depends on draw.io version 6.5.7 see application-diagram/pom.xml at application-diagram-1.2 · xwiki-contrib/application-diagram · GitHub
- The fact that there is a request for draw.io version 11.2.9 suggests that you have installed (also) the paid version, Diagram Application Pro (version 1.9) which depends on a more recent version of draw.io , see application-diagram/pom.xml at application-diagram-1.9 · xwikisas/application-diagram · GitHub
The free version of the Diagram Application doesn’t work with the more recent version of draw.io out of the box (there were lots of changes between version 6.5.7 and 11.2.9 as you can imagine).
If you want to use the free version then you should uninstall mxGraph client and editor, and draw.io extensions, then re-install Diagram Application 1.2 (which will bring the old version of draw.io that the free version of Diagram Application is compatible with). Also make sure you clear your browser cache afterwards.