Error #1009 when switching scene or frame.

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

          at map_fla::MainTimeline/limit()

          at map_fla::MainTimeline/moving()

here is my code

backS1.addEventListener(MouseEvent.MOUSE_DOWN, goS1);
function goS1(Event😞void{
        gotoAndStop(1, "Scene 1");
}

F5.addEventListener(MouseEvent.MOUSE_DOWN, gotoF5);
function gotoF5(Event😞void{
        gotoAndStop(5);
}

var toR:Boolean=false;
var toL:Boolean=false;
var toU:Boolean=false;
var toD:Boolean=false;

stage.addEventListener(KeyboardEvent.KEY_UP, notClick);
stage.addEventListener(KeyboardEvent.KEY_DOWN, onClick);
stage.addEventListener(Event.ENTER_FRAME, moving);

function onClick(event:KeyboardEvent😞void {
    switch (event.keyCode) {
    case Keyboard.RIGHT :
        toR=true;
    break;
    case Keyboard.LEFT :
        toL=true;
    break;
    case Keyboard.UP :
        toU=true;
    break;
    case Keyboard.DOWN :
        toD=true;
    break;
    }
}

function notClick(event:KeyboardEvent😞void {
    switch (event.keyCode) {
    case Keyboard.RIGHT :
        toR=false;
    break;
    case Keyboard.LEFT :
        toL=false;
    break;
    case Keyboard.UP :
        toU=false;
    break;
    case Keyboard.DOWN :
        toD=false;
    break;
    }
}

function moving(event:Event😞void {
    limit(map);
    if (toR) {
        map.x-=5;
    }
    if (toL) {
        map.x+=5;
    }
    if (toU) {
        map.y+=5;
    }
    if (toD) {
        map.y-=5;
    }
}   

navU.addEventListener(MouseEvent.MOUSE_DOWN, goU);
navD.addEventListener(MouseEvent.MOUSE_DOWN, goD);
navR.addEventListener(MouseEvent.MOUSE_DOWN, goR);
navL.addEventListener(MouseEvent.MOUSE_DOWN, goL);
navC.addEventListener(MouseEvent.MOUSE_DOWN, goC);

function goU(Event😞void{
    map.y += 25;
}
function goD(Event😞void{
    map.y -= 25;
}
function goR(Event😞void{
    map.x -= 25;
}
function goL(Event😞void{
    map.x += 25;
}
function goC(Event😞void{
    map.x = stage.stageWidth/2;
    map.y = stage.stageHeight/2;
}

map.addEventListener(MouseEvent.MOUSE_DOWN, Drag);
map.addEventListener(MouseEvent.MOUSE_UP, Drop);
map.addEventListener(MouseEvent.MOUSE_OUT, Drop);

function Drag(Event😞void {
    map.startDrag();
}

function Drop(Event😞void {
        map.stopDrag();
}  

function limit(o:MovieClip) {
   if (o.x + 364.5 < 500) {
       o.x = 500 - 364.5;
   }
   else
   if (o.x - 364.5 > 50) {
       o.x = 50 + 364.5;
   }  
   if (o.y + 309.5 < 375) {
       o.y = 375 - 309.5;
   }
   else
   if (o.y - 309.5 > 25) {
       o.y = 25 + 309.5;
   }
}

when I run my code, it's going well. the problem when I switch to another scene or frame it shows error #1009 and become lagging. but the code can still running. I dont know where the problem is. I'm totally noob. ty in advance.