Guard against undefined sessions in multiplayer

packages/node_modules/@node-red/runtime/lib/multiplayer/index.js

@@ -23,6 +23,7 @@ module.exports = {
             if (existingSessionId) {
                 connections.delete(opts.session)
                 const session = sessions.get(existingSessionId)
+                if (session) {
                     session.active = false
                     session.idleTimeout = setTimeout(() => {
                         sessions.delete(existingSessionId)
@@ -32,6 +33,7 @@ module.exports = {
                         data: { session: existingSessionId }
                     })
                 }
+            }
         })
         runtime.events.on('comms:message:multiplayer/connect', (opts) => {
             let session