NPE when restarting a wiki with a realtime editor session opened

Reproduction steps:

• Open a wysiwyg realtime editing session
• Restart the wiki

Expected result:

• The wiki is restarted on the backend, the UI probably displays that the session is lost during the time of the restart and manage to reconnect

Obtained result:

• the console log displays a NPE, and the UI displays the session is lost:

  2022-04-13 14:26:43.409:WARN :oejs.HttpChannel:qtp369049246-29: /xwiki/websocket/xwiki/netflux
  java.lang.NullPointerException
  	at org.xwiki.websocket.internal.DefaultWebSocketContext.createExecutionContext(DefaultWebSocketContext.java:142)
  	at org.xwiki.websocket.internal.DefaultWebSocketContext.initialize(DefaultWebSocketContext.java:78)
  	at org.xwiki.websocket.internal.XWikiEndpointConfigurator.modifyHandshake(XWikiEndpointConfigurator.java:68)
  	at org.eclipse.jetty.websocket.javax.server.internal.JavaxWebSocketCreator.createWebSocket(JavaxWebSocketCreator.java:153)