MP Create Game: don't allow proceeding if selected game has no sides

src/game_initialization/create_engine.cpp

@@ -352,6 +352,11 @@ void create_engine::init_generated_level_data()
 	//DBG_MP << current_level().data().debug();
 }
 
+bool create_engine::current_level_has_side_data()
+{
+	return current_level().data().has_child("side");
+}
+
 void create_engine::prepare_for_new_level()
 {
 	DBG_MP << "preparing mp_game_settings for new level\n";

src/game_initialization/create_engine.hpp

@@ -402,6 +402,9 @@ public:
 
 	saved_game& get_state();
 
+	/** Returns true if the current level has one or more [side] tags. */
+	bool current_level_has_side_data();
+
 private:
 	create_engine(const create_engine&) = delete;
 	void operator=(const create_engine&) = delete;

src/gui/dialogs/multiplayer/mp_create_game.cpp

@@ -712,7 +712,13 @@ void mp_create_game::load_game_callback(window& window)
 	window.set_retval(LOAD_GAME);
 }
 
-bool mp_create_game::dialog_exit_hook(window&) {
+bool mp_create_game::dialog_exit_hook(window& window)
+{
+	if(!create_engine_.current_level_has_side_data()) {
+		gui2::show_transient_error_message(window.video(), _("The selected game has no sides!"));
+		return false;
+	}
+
 	if(create_engine_.current_level_type() != ng::level::TYPE::CAMPAIGN &&
 	   create_engine_.current_level_type() != ng::level::TYPE::SP_CAMPAIGN
 	) {