Cannot make a line through TouchEvent

I know make a line could be create in MouseEvent and Event. But, those of them has a weakness such as Multitouch is not supported in AIR mobile. Then, I migrate those code to TouchEvent. But, It's make me frustated because these problem always block me out.

Output:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
  at Untitled_fla::MainTimeline/this_move()[Untitled_fla.MainTimeline::frame1:32]
  at runtime::ContentPlayer/simulationSendTouchEvent()
  at runtime::SimulatedContentPlayer/clientSocketDataHandler()
  at runtime::ContentPlayer/simulationSendTouchEvent()
  at runtime::SimulatedContentPlayer/clientSocketDataHandler()

Here's the code:

import flash.display.MovieClip;
import flash.display.Graphics;
import flash.events.TouchEvent;
import flash.geom.ColorTransform;
import flash.events.Event;


var lastX:Number
var lastY:Number
var drawing:Boolean = false


Multitouch.inputMode = MultitouchInputMode.TOUCH_POINT
stage.addEventListener(TouchEvent.TOUCH_BEGIN, this_down)
stage.addEventListener(TouchEvent.TOUCH_MOVE, this_move)
stage.addEventListener(TouchEvent.TOUCH_END, touch_up)


function this_down(e:TouchEvent)
{
     lastX = e.localX
     lastY = e.localY
     drawing = true
}


function this_move(e:TouchEvent)
{
     if(drawing)
     {
          this.graphic.lineStyle(20,0x000000)
          this.drawArea.graphic.moveTo(lastX,lastY)
          this.drawArea.graphic.lineTo(e.localX,e.localY)
          lastX = e.localX
           lastY = e.localY
           e.updateAfterEvent()
     }
}


function touch_up(event:TouchEvent)
{
     drawing = false
}