AS3 - TypeError # 1009 with Timers - actionscript-3

Question

I just have no idea what to do with this. I've been looking through this for an hour now and I keep getting an error reading:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Shooter_Enemy/shotHandler()
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()
When I debug it it points to the line of the code where "seekingBullet" is added to the stage. Any help resolving this would be greatly welcomed.
package
{
import flash.display.*;
import flash.events.*;
import flash.utils.Timer;
public class Shooter_Enemy extends MovieClip
{
private var yMove:int = 2;
private var shootTimer:Timer = new Timer(500);
public function Shooter_Enemy()
{
this.name = "mc_shooter_enemy";
this.addEventListener(Event.ENTER_FRAME,enemyMove);
shootTimer.start();
shootTimer.addEventListener(TimerEvent.TIMER,shotHandler);
}
public function addShooterEnemy(X:int):void
{
this.x = X;
this.y = 0;
}
public function removeEnemy()
{
shootTimer.removeEventListener(TimerEvent.TIMER,shotHandler);
shootTimer.stop();
this.removeEventListener(Event.ENTER_FRAME,enemyMove);
this.x = 0;
this.y = (stage.height + this.height);
}
private function shotHandler(te:TimerEvent):void
{
var seekingBullet:SeekingBullet = new SeekingBullet();
Main.seekingBulletArray.push(seekingBullet);
stage.addChild(seekingBullet);
seekingBullet.addSeekingBullet(this.x,this.y);
}
private function enemyMove(e:Event)
{
this.y += yMove;
}
}
}

Answer 1

If it's actual, a good practice for using stage is listening *Event.ADDED_TO_STAGE* and then starting your activity like:
package
{
import flash.display.*;
import flash.events.*;
import flash.utils.Timer;
public class Shooter_Enemy extends MovieClip
{
private var yMove:int = 2;
private var shootTimer:Timer = new Timer(500);
public function Shooter_Enemy()
{
this.name = "mc_shooter_enemy";
this.addEventListener(Event.ENTER_FRAME,enemyMove);
shootTimer.addEventListener(TimerEvent.TIMER, shotHandler);
addEventListener(Event.ADDED_TO_STAGE, addedToStageHandler);
}
public function addShooterEnemy(X:int):void
{
this.x = X;
this.y = 0;
}
public function removeEnemy()
{
shootTimer.removeEventListener(TimerEvent.TIMER,shotHandler);
shootTimer.stop();
this.removeEventListener(Event.ENTER_FRAME,enemyMove);
this.x = 0;
this.y = (stage.height + this.height);
}
private function addedToStageHandler(e:Event)
{
shootTimer.start();
}
private function shotHandler(te:TimerEvent):void
{
var seekingBullet:SeekingBullet = new SeekingBullet();
Main.seekingBulletArray.push(seekingBullet);
stage.addChild(seekingBullet);
seekingBullet.addSeekingBullet(this.x,this.y);
}
private function enemyMove(e:Event)
{
this.y += yMove;
}
}
}