TypeError: Error #1009: Cannot access a property or method of a null object reference

For some reason when ever I add the Mouse event listener I get the #1009 error and some stuff dissapears and the clicking function wont work, can anyone help?

package

{

          import flash.display.*;

          import flash.text.*;

          import flash.events.MouseEvent;

          import flash.events.KeyboardEvent;

          import flash.ui.*;

          import flash.utils.*;

          import flash.geom.*;

          import flash.net.*;

          import flash.media.Sound;

          import flash.media.SoundTransform;

          import flash.media.SoundChannel;

          import flash.net.URLRequest;

          import flash.display.Bitmap;

          import flash.display.Sprite;

          import flash.events.Event;

          import flash.utils.Timer;

          import flash.events.TimerEvent;

          public class Platforms extends Sprite

          {

                    var Floor = new floor();

                    var HighLightBox=new highLightBox();

                    var nyEditor:int = 0;

                    var nxEditor:int = 0;

                    var ExFloor=new exFloor();

                    var tFloor:exFloor = new exFloor  ;

                    var bEditorClick:Boolean = false;

                    var arFloor = new Array();

                    var tmrBoxes:Timer = new Timer(35);

                    public function Platforms()

                    {

                              addChild(ExFloor);

                              addChild(Floor);

                              Floor.y = 500;

                              Floor.x = 300;

                              // constructor code

                             stage.addEventListener(MouseEvent.CLICK,Clicking);

                              tmrBoxes.addEventListener("timer",timerBoxes);

                              tmrBoxes.start();

                    }

                    public function TouchingGround(HeroGroundTouch:Sprite):Boolean

                    {

                              if (HeroGroundTouch.hitTestObject(Floor))

                              {

                                        return true;

                              }

                              return false;

                    }

                    public function Clicking(e:MouseEvent):void

                    {

                              if (bEditorClick==false)

                              {

                                        nxEditor = mouseX;

                                        nyEditor = mouseY;

                                        addChild(HighLightBox);

                                        HighLightBox.x = mouseX;

                                        HighLightBox.y = mouseY;

                                        bEditorClick = true;

                              }

                              else

                              {

                                        trace("            PlaceFloor("+nxEditor+","+nyEditor+","+(mouseX-nxEditor)+","+(mouseY-nyEditor)+");");

                                        PlaceFloor(nxEditor,nyEditor,(mouseX-nxEditor),(mouseY-nyEditor));

                                        removeChild(HighLightBox);

                                        bEditorClick = false;

                              }

                    }

                    public function PlaceFloor(nx:int,ny:int,nWidth:int,nHeight:int):void

                    {

                              tFloor=new exFloor();

                              addChild(tFloor);

                              arFloor.push(tFloor);

                              tFloor.x = nx;

                              tFloor.y = ny;

                              tFloor.width = nWidth;

                              tFloor.height = nHeight;

                    }

                    function timerBoxes(eventArgs:TimerEvent)

                    {

                              HighLightBox.width = mouseX - HighLightBox.x;

                              HighLightBox.height = mouseY - HighLightBox.y;

                    }

          }

}