Handle case where we don't have the occupant

It can now happen because we also auto-complete for nicks based on
messages in the chat

src/converse-muc-views.js

@@ -1386,8 +1386,12 @@ converse.plugins.add('converse-muc-views', {
                     return false;
                 }
                 const reason = args.split(nick_or_jid, 2)[1].trim();
-                // We're guaranteed to have an occupant due to getNickOrJIDFromCommandArgs
                 const occupant = this.model.getOccupant(nick_or_jid);
+                if (!occupant) {
+                    this.showErrorMessage(__("Couldn't find a participant with that nickname or JID. They might have left the groupchat."));
+                    return;
+                }
+
                 const attrs = {
                     'jid': occupant.get('jid'),
                     'reason': reason