Question

I followed the example from a previous question and I am loading an external swf using a loader and inside the loader event handler I am trying to cast the loader.content as my custom class PanelReferenceClip which extends MovieClip

When I publish I receive a this error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

Just to make sure and test that the swf location was correct and the swf was actually being loaded, I changed the type of the content to as MovieClip and it worked fine.

EDIT: I also wanted to add that these swfs are being stored locally and not being pulled across the internet, multiple networks or servers.

I am not sure if I did something quirky in my class so I am providing the source to my custom class PanelReferenceClip

package com.components 
{
    import com.UI.DevicePanel;
    import flash.display.MovieClip;

    /**
     * ...
     * 
     * used to store the loaded swf inside the panel
     * 
     * *parentPanel is set so that it is able to access it's parent Panel when needing
     * to set parameters.
     */

    public class PanelReferenceClip extends MovieClip 
    {
        private var _parentPanel:DevicePanel;
        private var _bg_mc:MovieClip;
        private var _oldY:Number = 0;
        private var _oldX:Number = 0;
        private var _IsDragging:Boolean = false;

        public function PanelReferenceClip() {
            super();
        }

        /*--------------------------------------------------------------------------
         * GETTERS AND SETTERS
         * -----------------------------------------------------------------------*/

        public function set parentPanel(p:DevicePanel):void {
            _parentPanel = p;
        }

        public function get parentPanel():DevicePanel {
            return _parentPanel;
        }

        public function get bg_mc():MovieClip {
            try {
                return getChildByName("bg_mc") as MovieClip;
            } catch (e:Error) {
                trace("could not find bg_mc in " + _parentPanel.DeviceName + " panel");
            }

            return null;
        }

        public function set oldY(n:Number):void {
            _oldY = n;
        }

        public function get oldY():Number {
            return _oldY;
        }

        public function set oldX(n:Number):void {
            _oldX = n;
        }

        public function get oldX():Number {
            return _oldX;
        }

        public function set IsDragging(b:Boolean):void {
            _IsDragging = b;
        }

        public function get IsDragging():Boolean {
            return _IsDragging;
        }
    }

}

Here is the part of another class that is loading the swfs and then trying to assign them as the class prop _reference which is of type PanelReferenceClip . I am doing this so I am able to get ahold of the swf and it’s children because when you import a swf you do not get to set the instance name of the imported swf. So I am assigning it a custom class that extends MovieClip so I can store have some custom properties.

        private function handleLoad(e:Event):void
        {
            e.target.removeEventListener(Event.COMPLETE, handleLoad, false);
            // keep reference to the content
            _reference = e.target.content as PanelReferenceClip;
                    //  ** BREAKS ON THE NEXT LINE **/
            trace(_reference.numChildren);

            // add loader to the display list so we can see the external SWF.
            addChild(e.target.loader);

            // signal the sim engine that the swf has loaded 
            // and to go ahead and wire up the components
            dispatchEvent(new DataEvent(DataEvent.COMPLETE));

            initPanel();
        }

Here is the method used to load the swf. I added in the application context part to try it out but I am still not getting anywhere.

    public function loadSWF(theSWF:String):void
    {
        var url:String = theSWF;
        var urlReq:URLRequest = new URLRequest(url);
        _urlError = url;
        _loader.contentLoaderInfo.addEventListener(Event.COMPLETE, handleLoad);
        _loader.contentLoaderInfo.addEventListener(IOErrorEvent.IO_ERROR, ioErrorHandler);
        var context:LoaderContext = new LoaderContext(false, ApplicationDomain.currentDomain);
        _loader.load(urlReq,context);
        _loader.mouseEnabled = false;
    }

Answer 1

This might be caused by converting between types that can’t be typecast. Instead of an error occurring when performing someVariable as OtherClass, the variable will just become null. (Example Shown)

I would store a reference to the original movieclip as one of the properties in PanelReferenceClip, and just use that reference when you need to access things like .numChildren

//MovieClips are subclasses of Sprites... this conversion works
var originalMC:MovieClip = new MovieClip();
var sprite:Sprite = originalMC as Sprite; 
trace("sprite: " + sprite); //defined

//Sprites are not subclasses of MovieClips... this conversion wont work
var originalSprite:Sprite = new Sprite();
var mc:MovieClip = originalSprite as MovieClip; 
trace("mc: " + mc); //null

//MovieClips are not subclasses of PanelReferenceClips (quite the opposite)
//this conversion wont work, just as the one before
var panelRef:PanelReferenceClip = originalMC as PanelReferenceClip;
trace("panelRef: " + panelRef); //null