FileStream openAsync throws Error #1009

Hi AIR devs.

Have a problem regarding FileStream openAsync

here my code

    var file:File = File.applicationStorageDirectory.resolvePath(fName+'.'+EXT);
    var fileStream:FileStream = new FileStream();  
     if (!file.exists) {
              this.dispatchEvent(new AppEvent(AppEvent.DATA, null, false));                                         
     }else {
         fileStream.addEventListener(Event.COMPLETE, fileReadCompleteHandler);
         fileStream.openAsync(file, FileMode.READ);
         fileStream.addEventListener(Event.CLOSE, fileClosedHandler);
         fileStream.addEventListener(IOErrorEvent.IO_ERROR, IOErrorHandler);
     }
     private function fileReadCompleteHandler(event:Event):void {
        var ob:Object;
        var fileStream:FileStream = FileStream(event.currentTarget);
        try {
            ob.source = fileStream.readObject();
        }catch (e:Error) {
            trace('error:' + e.message) 
        }
        fileStream.removeEventListener(Event.COMPLETE, fileReadCompleteHandler);
        fileStream.close(); 
      }

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

what I am missing? fileStream.open(file, FileMode.READ) works as expected.