Steps to reproduce :
- Create an UI extension and mark it as "async", you may want to select all the available "context elements" in the UIX Object, just to make sure that the bug does not come from a missing context element
- In the content of this UI extension, make a call to a CSS file in the war, in my case I used #set($discard = $services.template.execute('logging_macros.vm'))
- Save and view ; see if the UIX is rendered (in theory it's not)
- If you look carefully at the network logs, you'll see that the rendering of the UI components failed
Looking at the issue, this is caused by a NullPointerException here : https://github.com/xwiki/xwiki-platform/blob/5509394d8343194f722273af4c29944bfe9a2857/xwiki-platform-core/xwiki-platform-skin/xwiki-platform-skin-skinx/src/main/java/com/xpn/xwiki/plugin/skinx/CssSkinFileExtensionPlugin.java#L74 ; getParametersForResource(filename, context) will not contain any key forceSkinAction.