[undefined]

забыл добавить выставление curScreen в showScreen

package 
{
	import flash.display.DisplayObjectContainer;
	/**
	 * ...
	 * @author 
	 */
	public class DisplayManager 
	{
		private static var root:DisplayObjectContainer;
		private static var screens:Array;
		private static var curScreen:int =-1;
		public static function init(_root:DisplayObjectContainer):void {
			root = _root;
			screens = [];
		}
		public static function addScreen(screen:DisplayObjectContainer):void {
			screens.push(screen);
		}
		public static function showScreen(n:int):void {
			if (curScreen !=-1 && screens && screens[curScreen].parent) {
				screens[curScreen].parent.removeChild(screens[curScreen]);
			}
			if (screens[n]) {
				root.addChild(screens[n]);
				curScreen = n;
			}
		}
		public static function get rootDO():DisplayObjectContainer {
			return root;
		}
 
	}
 
}

поправь у себя