Access of undefined property errors - actionscript-3

Question

I'm getting quite a few errors after I started learning AS3 (Trying to dynamically add the crosshair to the stage instead of using mouse cursor for shooter game):
Main.as, Line 13 1180: Call to a possibly undefined method addChild.
Main.as, Line 13 1120: Access of undefined property crosshair.
Main.as, Line 20 1120: Access of undefined property stage.
Main.as, Line 20 1120: Access of undefined property moveCursor.

Answer 1

You have no constructor in your class.
try this :
package {
import flash.display.MovieClip;
import flash.events.Event;
import flash.events.MouseEvent;
import flash.ui.Mouse;
import flash.media.Sound;
import flash.media.SoundChannel;
public class Main extends MovieClip {
public var crosshair:crosshair_mc;
public function Main()
{
//This creates a new instance of the cursor movie clip and adds it onto
//the stage using the addChild method.
crosshair = new crosshair_mc();
addChild(crosshair);
//Hides the default cursor on the stage so it will not be shown.
Mouse.hide();
//Adds an event listener onto the stage with the enter frame event which
//repeatedly executes the moveCursor function.
stage.addEventListener(Event.ENTER_FRAME, moveCursor);
}
//This function set the x & y positions of the custom cursor to the x & y positions
//of the default cursor.
function moveCursor(event:Event)
{
crosshair.x=mouseX;
crosshair.y=mouseY;
}
}
}