Action script 3.0 output error please help flash pro

So, I am creating a type effect animation in flash and my code for the first frame in my time line is;

1. var i=0;
2. var myString:String = "Name: Earth\nDiameter: 12,756 km"
4. var mytimer:Timer = new Timer(200,myString.length);
5. mytimer.addEventListener(TimerEvent.TIMER, timerHandler);
6. mytimer.start();
8. function timerHandler(event:TimerEvent):void{
9.           mytext.appendText (myString.charAt(i));
10.           i++;
11.           if (i==myString.length && currentFrame == 1){ gotoAndStop(10)}
12. }

My second code on the next frame is essentailly the same thing but becuase i cannot place a var again I have taken them out and put code that refs the first code;

1. i=0;
3. mytext.text = ""; // empty the textfield
6. myString = "Name: Neptune\nDiameter: 49,532 km";
8. mytimer.repeatCount = myString.length;
9. mytimer.reset();
10. mytimer.start();

My animation still works correctly as I wanted however if i then place the above code on another frame at time line i receive an output error code refusing it to show the third frame;

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

          at Animation_fla::MainTimeline/timerHandler()

          at flash.utils::Timer/_timerDispatch()

          at flash.utils::Timer/tick()

What does this mean can someone explain it to me please and how I would fix ti?

Thanks