Fix crash when trying to join games in lobby dialog

This fixes an oversight from d655455304. During a code deduplication, a
check was removed. When looking at the lobby as a dialog (while in
gamesetup), it should be impossible to try to join a game.

binaries/data/mods/public/gui/lobby/LobbyPage/Buttons/JoinButton.js

@@ -67,6 +67,9 @@ class JoinButton
 				return;
 		}
 
+		if (this.joinButton.hidden)
+			return;
+
 		Engine.PushGuiPage("page_gamesetup_mp.xml", {
 			"multiplayerGameType": "join",
 			"name": g_Nickname,