Encountered this when redeploying members which used a message type not registered on previous members:

java.lang.NullPointerException: Cannot invoke "java.util.Map.get(Object)" because "this.map" is null
at org.jgroups.DefaultMessageFactory.create(DefaultMessageFactory.java:31)

Here's a PR:

https://github.com/belaban/JGroups/pull/789

This will instead make it throw the expected IAE "no creator found for type".