Fixed AssetViewer crashing when loading a scripted asset

lib/ui/menus/level_select_menu.rb

@@ -7,6 +7,8 @@ class IMICFPS
       subtitle "Choose a Map"
 
       Dir.glob("#{GAME_ROOT_PATH}/maps/*.json").map { |file| [file, MapParser.new(map_file: file)] }.each do |file, map|
+        next unless map.metadata.gamemode
+
         link map.metadata.name do
           push_state(
             LoadingState.new(forward: Game, map_file: file)