When using Keycloak as a WS-Fed identity broker, single logout leads to a NullPointerException

[AlistairDoswald]

This can be tested with the following setup:

Client (WS-Fed)<---> Keycloak (Identity broker, WS-Fed) <---> Keycloak (Exernal IDP, WS-Fed)

With all elements set up correctly for Single Sign-on and logout, we can have a NullPointerException. The user is signed out of the external IDP, but neither from the identity broker nor its clients.

The cause of the NullPointerException must be verified and handled. The expected behaviour is that the user is signed out from the IDP and all of its clients.

The case should be tested with all combinations of Backchannel Logout and Handle Empty Action as wsignoutcleanup1.0 set to on/off.