Error code in Actions Script for Animate

hello.  i'm digital art teacher , have been using tutorials website create animate files.  unfortunately 1 of tutorials causing error of students.  following code directed use , how create actions frame.

 

package  {

    import flash.utils.timer;

    import flash.text.textfield;

    import flash.events.timerevent;

 

public class typefx {

 

    private var timer:timer;

    private var text:string;

    private var pos:int = 0;

    private var field:textfield;

 

    public function typefx(field:textfield, speed:int = 42, text:string = null) {

        // constructor code

 

        this.field = field;

 

        if(text != null)

        {

            this.text = text;

        }

        else

        {

            this.text = field.text;

        }

 

        field.text = '';

 

        timer = new timer(speed, this.text.length);

        timer.addeventlistener(timerevent.timer,update);

        timer.addeventlistener(timerevent.timer_complete,kill);

        timer.start();

    }

 

    private function update(e:timerevent):void

    {

        pos++;

        field.text = text.substr(0,pos);

    }

 

    private function kill(e:timerevent):void

    {

        timer.removeeventlistener(timerevent.timer,update);

        timer.removeeventlistener(timerevent.timer_complete,kill);

        timer.stop();

        timer = null;

        text = null;

        field = null;

    }

 

    }

 

}

 

on actions frames instructed put following:

new typefx(textbox_txt);

 

the error message received following:

scene 1, layer 'actions', frame 1, line 3, column 12

1120: access of undefined property textbox_txt.

there import line may not have shown, like:

 

import typefx;

 

that import external typefx.as file showed code for.

 

the other thing textfield you're going add effects need dynamic textfield, instance name of textbox_txt. selecting textfield on stage, , in properties panel give name.

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