动作脚本3错误1009

Question

package 
{
    import flash.display.MovieClip;
    public class DocumentClass extends MovieClip 
    {
        public var playScreen:AvoiderGame;

        public function DocumentClass() 
        {
            playScreen = new AvoiderGame();
            addChild( playScreen );
        }
    }
}

获取此信息：

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at AvoiderGame()
    at DocumentClass()