Draw Canvas problem with javascript (plugin)

I really surprised...

My plugin is worked very well in build r95..

Problem comes after I attempt to update to r114.

Javascript library don't recognize the context of element...???

My code as bellow:

<font color=RED>

     instanceProto.paint = function (myparam)

     {

      //alert("here: "+ this.runtime.id);      

      //alert(this.x);

      var _x = this.x;

      var _y = this.y;

      var _w = this.width;

      var _h = this.height;

        //this.runtime.trigger(pluginProto.cnds.ImageLoaded,instance);

        var image = new Image();

        image.src = myparam;               

        image.onload= function(){

            var c=document.getElementById("c2canvas");

            var con=c.getContext('2d');

            if(null == con){

               alert("con is null");

               return;

            }

            con.drawImage(image, _x - _w/2, _y- _h/2, _w,_h);

        }

           //this.draw(con);

           //this.runtime.redraw=true;

        this.image = image;

     };

</font>

In test bed, alert message always topup "con is null"...this problem never appeared in old build

Some one please help me...how to solve this problem

Thank in advance