A bad event descriptor can prevent all events to be dispatched to users

I got the following error log on xwiki.org:

2022-03-17 14:08:42,020 [Asynchronous handler for event store [solr]] ERROR .o.i.DefaultObservationManager - Failed to send event [org.xwiki.eventstream.events.EventStreamAddedEvent@473b0d46] to listener [org.xwiki.notifications.notifiers.internal.UserEventDispatcherListener@6b22c1df]
java.lang.NullPointerException: null
at org.xwiki.notifications.notifiers.internal.UserEventDispatcherListener.prefilterEvent(UserEventDispatcherListener.java:126)
at org.xwiki.notifications.notifiers.internal.UserEventDispatcherListener.onEvent(UserEventDispatcherListener.java:106)
at org.xwiki.observation.internal.DefaultObservationManager.notify(DefaultObservationManager.java:320)
at org.xwiki.observation.internal.DefaultObservationManager.notify(DefaultObservationManager.java:285)
at org.xwiki.observation.internal.DefaultObservationManager.notify(DefaultObservationManager.java:337)
at org.xwiki.eventstream.internal.AbstractAsynchronousEventStore.complete(AbstractAsynchronousEventStore.java:441)
at org.xwiki.eventstream.internal.AbstractAsynchronousEventStore.afterTasks(AbstractAsynchronousEventStore.java:523)
at org.xwiki.eventstream.store.solr.internal.SolrEventStore.afterTasks(SolrEventStore.java:170)
at org.xwiki.eventstream.internal.AbstractAsynchronousEventStore.processTasks(AbstractAsynchronousEventStore.java:347)
at org.xwiki.eventstream.internal.AbstractAsynchronousEventStore.run(AbstractAsynchronousEventStore.java:325)
at org.glowroot.agent.plugin.executor.RunnableWrapper.run(RunnableWrapper.java:35)
at java.base/java.lang.Thread.run(Thread.java:829)

which suggest we have a bad descriptor which return null to RecordableEventDescriptor#getEventType().

If this descriptor ends up being one of the first descriptor found, it will prevent most of the events to be dispatched to users.