Missing throbber.gif caused error on the logs

How to reproduce:

• A crawler visiting a page that includes tree.min.css (e.g.:

  https://<server>/view/Macros/Tree

  that requests

   https://<server>/webjars/wiki%3Axwiki/xwiki-platform-tree-webjar/12.10.10/tree.min.css?evaluate=true

• The crawler will parse the CSS to find images included in the CSS and will find

   background:url("/webjars/wiki%3Axwiki/xwiki-platform-tree-webjar/12.10.10/throbber.gif") 

  in tree.min.css

• The crawler tries to load

   https://<server>/webjars/wiki%3Axwiki/xwiki-platform-tree-webjar/12.10.10/throbber.gif

  to download the image

Result: the following error is printed on the logs

2022_11_03.jetty.log:2022-11-03 10:25:31,553 [qtp2025864991-292030 - https://<server>/webjars/wiki%3Axwiki/xwiki-platform-tree-webjar/12.10.10/throbber.gif] ERROR o.i.ObservationContextListener - Can't find any begin event corresponding to [class org.xwiki.resource.events.ResourceReferenceHandledEvent (type = [webjars], parameters = [])] 

Expected result: no error is printed on the server logs