Error #2007 in Flash CS5.5

Question

I'm currently learning on creating a puzzle game using Flash CS5.5 and AS3.0. I've created the puzzle on different scenes. However, when I Test Movie (Ctrl+Enter), it says;

TypeError: Error #2007: Parameter hitTestObject must be non-null.
    at flash.display::DisplayObject/_hitTest()
    at flash.display::DisplayObject/hitTestObject()
    at RECOVER_MAIN__fla::MainTimeline/Ent2Frame()

What does it means? How can I fix this error? AS3.0 of my puzzle as shown below. Thank you.

Puzzle 1(6 pieces):

import flash.events.MouseEvent;

stage.addEventListener(Event.ADDED_TO_STAGE,Ent1Frame);

function Ent1Frame(e:Event):void
{
    //piece1
    PQ1piece1_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP1)
    function DragP1(event:MouseEvent):void
    {
        PQ1piece1_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP1)
    function DropP1(event:MouseEvent):void
    {
        PQ1piece1_mc.stopDrag();
    }

    if(Targ1_mc.hitTestObject(PQ1piece1_mc.Tar1_mc))
    {
        PQ1piece1_mc.x = 149.65;
        PQ1piece1_mc.y = 150.30;
    }

    //piece2
    PQ1piece2_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP2)
    function DragP2(event:MouseEvent):void
    {
        PQ1piece2_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP2)
    function DropP2(event:MouseEvent):void
    {
        PQ1piece2_mc.stopDrag();
    }

    if(Targ2_mc.hitTestObject(PQ1piece2_mc.Tar2_mc))
    {
        PQ1piece2_mc.x = 283.15;
        PQ1piece2_mc.y = 150.30;
    }

    //piece3_mc
    PQ1piece3_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP3)
    function DragP3(event:MouseEvent):void
    {
        PQ1piece3_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP3)
    function DropP3(event:MouseEvent):void
    {
        PQ1piece3_mc.stopDrag();
    }

    if(Targ3_mc.hitTestObject(PQ1piece3_mc.Tar3_mc))
    {
        PQ1piece3_mc.x = 416.65;
        PQ1piece3_mc.y = 150.30;
    }

    //PQ1piece4_mc
    PQ1piece4_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP4)
    function DragP4(event:MouseEvent):void
    {
        PQ1piece4_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP4)
    function DropP4(event:MouseEvent):void
    {
        PQ1piece4_mc.stopDrag();
    }

    if(Targ4_mc.hitTestObject(PQ1piece4_mc.Tar4_mc))
    {
        PQ1piece4_mc.x = 149.65;
        PQ1piece4_mc.y = 350.30;
    }

    //PQ1piece5_mc
    PQ1piece5_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP5)
    function DragP5(event:MouseEvent):void
    {
        PQ1piece5_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP5)
    function DropP5(event:MouseEvent):void
    {
        PQ1piece5_mc.stopDrag();
    }

    if(Targ5_mc.hitTestObject(PQ1piece5_mc.Tar5_mc))
    {
        PQ1piece5_mc.x = 283.15;
        PQ1piece5_mc.y = 350.30;
    }

    //PQ1piece6_mc
    PQ1piece6_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP6)
    function DragP6(event:MouseEvent):void
    {
        PQ1piece6_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP6)
    function DropP6(event:MouseEvent):void
    {
        PQ1piece6_mc.stopDrag();
    }

    if(Targ6_mc.hitTestObject(PQ1piece6_mc.Tar6_mc))
    {
        PQ1piece6_mc.x = 415.35;
        PQ1piece6_mc.y = 348.80;
    }
}

---

Puzzle 2(16 pieces):

import flash.events.MouseEvent;

stage.addEventListener(Event.ENTER_FRAME,Ent2Frame);

function Ent2Frame(e:Event):void
{
    //piece1
    PQ2piece1_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP7)
    function DragP7(event:MouseEvent):void
    {
        PQ2piece1_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP7)
    function DropP7(event:MouseEvent):void
    {
        PQ2piece1_mc.stopDrag();
    }

    if(Targ7_mc.hitTestObject(PQ2piece1_mc.Tar7_mc))
    {
        PQ2piece1_mc.x = 132.20;
        PQ2piece1_mc.y = 99.05;
    }

    //piece2
    PQ2piece2_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP8)
    function DragP8(event:MouseEvent):void
    {
        PQ2piece2_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP8)
    function DropP8(event:MouseEvent):void
    {
        PQ2piece2_mc.stopDrag();
    }

    if(Targ8_mc.hitTestObject(PQ2piece2_mc.Tar8_mc))
    {
        PQ2piece2_mc.x = 232.20;
        PQ2piece2_mc.y = 99.05;
    }

    //piece3_mc
    PQ2piece3_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP9)
    function DragP9(event:MouseEvent):void
    {
        PQ2piece3_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP9)
    function DropP9(event:MouseEvent):void
    {
        PQ2piece3_mc.stopDrag();
    }

    if(Targ9_mc.hitTestObject(PQ2piece3_mc.Tar9_mc))
    {
        PQ2piece3_mc.x = 332.20;
        PQ2piece3_mc.y = 99.05;
    }

    //PQ1piece4_mc
    PQ2piece4_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP10)
    function DragP10(event:MouseEvent):void
    {
        PQ2piece4_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP10)
    function DropP10(event:MouseEvent):void
    {
        PQ2piece4_mc.stopDrag();
    }

    if(Targ10_mc.hitTestObject(PQ2piece4_mc.Tar10_mc))
    {
        PQ2piece4_mc.x = 432.30;
        PQ2piece4_mc.y = 99.05;
    }

    //piece5_mc
    PQ2piece5_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP11)
    function DragP11(event:MouseEvent):void
    {
        PQ2piece5_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP11)
    function DropP11(event:MouseEvent):void
    {
        PQ2piece5_mc.stopDrag();
    }

    if(Targ11_mc.hitTestObject(PQ2piece5_mc.Tar11_mc))
    {
        PQ2piece5_mc.x = 132.20;
        PQ2piece5_mc.y = 199.05;
    }

    //PQ1piece6_mc
    PQ2piece6_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP12)
    function DragP12(event:MouseEvent):void
    {
        PQ2piece6_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP12)
    function DropP12(event:MouseEvent):void
    {
        PQ2piece6_mc.stopDrag();
    }

    if(Targ12_mc.hitTestObject(PQ2piece6_mc.Tar12_mc))
    {
        PQ2piece6_mc.x = 232.20;
        PQ2piece6_mc.y = 199.05;
    }

    //piece7_mc
    PQ2piece7_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP13)
    function DragP13(event:MouseEvent):void
    {
        PQ2piece7_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP13)
    function DropP13(event:MouseEvent):void
    {
        PQ2piece7_mc.stopDrag();
    }

    if(Targ13_mc.hitTestObject(PQ2piece7_mc.Tar13_mc))
    {
        PQ2piece7_mc.x = 332.20;
        PQ2piece7_mc.y = 199.05;
    }

    //piece8_mc
    PQ2piece8_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP14)
    function DragP14(event:MouseEvent):void
    {
        PQ2piece8_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP14)
    function DropP14(event:MouseEvent):void
    {
        PQ2piece8_mc.stopDrag();
    }

    if(Targ14_mc.hitTestObject(PQ2piece8_mc.Tar14_mc))
    {
        PQ2piece8_mc.x = 432.20;
        PQ2piece8_mc.y = 199.05;
    }

    //piece9_mc
    PQ2piece9_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP15)
    function DragP15(event:MouseEvent):void
    {
        PQ2piece9_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP15)
    function DropP15(event:MouseEvent):void
    {
        PQ2piece9_mc.stopDrag();
    }

    if(Targ15_mc.hitTestObject(PQ2piece9_mc.Tar15_mc))
    {
        PQ2piece9_mc.x = 132.20;
        PQ2piece9_mc.y = 299.05;
    }

    //piece10_mc
    PQ2piece10_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP16)
    function DragP16(event:MouseEvent):void
    {
        PQ2piece10_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP16)
    function DropP16(event:MouseEvent):void
    {
        PQ2piece10_mc.stopDrag();
    }

    if(Targ16_mc.hitTestObject(PQ2piece10_mc.Tar16_mc))
    {
        PQ2piece10_mc.x = 232.20;
        PQ2piece10_mc.y = 299.05;
    }

    //piece11_mc
    PQ2piece11_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP17)
    function DragP17(event:MouseEvent):void
    {
        PQ2piece11_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP17)
    function DropP17(event:MouseEvent):void
    {
        PQ2piece11_mc.stopDrag();
    }

    if(Targ17_mc.hitTestObject(PQ2piece11_mc.Tar17_mc))
    {
        PQ2piece11_mc.x = 332.20;
        PQ2piece11_mc.y = 299.05;
    }

    //piece12_mc
    PQ2piece12_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP18)
    function DragP18(event:MouseEvent):void
    {
        PQ2piece12_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP18)
    function DropP18(event:MouseEvent):void
    {
        PQ2piece12_mc.stopDrag();
    }

    if(Targ18_mc.hitTestObject(PQ2piece12_mc.Tar18_mc))
    {
        PQ2piece12_mc.x = 432.20;
        PQ2piece12_mc.y = 299.05;
    }

    //piece13_mc
    PQ2piece13_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP19)
    function DragP19(event:MouseEvent):void
    {
        PQ2piece13_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP19)
    function DropP19(event:MouseEvent):void
    {
        PQ2piece13_mc.stopDrag();
    }

    if(Targ19_mc.hitTestObject(PQ2piece13_mc.Tar19_mc))
    {
        PQ2piece13_mc.x = 132.20;
        PQ2piece13_mc.y = 398.40;
    }

    //piece14_mc
    PQ2piece14_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP20)
    function DragP20(event:MouseEvent):void
    {
        PQ2piece14_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP20)
    function DropP20(event:MouseEvent):void
    {
        PQ2piece14_mc.stopDrag();
    }

    if(Targ20_mc.hitTestObject(PQ2piece14_mc.Tar20_mc))
    {
        PQ2piece14_mc.x = 232.20;
        PQ2piece14_mc.y = 398.40;
    }

    //piece15_mc
    PQ2piece15_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP21)
    function DragP21(event:MouseEvent):void
    {
        PQ2piece15_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP21)
    function DropP21(event:MouseEvent):void
    {
        PQ2piece15_mc.stopDrag();
    }

    if(Targ21_mc.hitTestObject(PQ2piece15_mc.Tar21_mc))
    {
        PQ2piece15_mc.x = 332.20;
        PQ2piece15_mc.y = 398.40;
    }

    //piece16_mc
    PQ2piece16_mc.addEventListener(MouseEvent.MOUSE_DOWN,DragP22)
    function DragP22(event:MouseEvent):void
    {
        PQ2piece16_mc.startDrag();
    }

    stage.addEventListener(MouseEvent.MOUSE_UP,DropP22)
    function DropP22(event:MouseEvent):void
    {
        PQ2piece16_mc.stopDrag();
    }

    if(Targ22_mc.hitTestObject(PQ2piece16_mc.Tar22_mc))
    {
        PQ2piece16_mc.x = 432.20;
        PQ2piece16_mc.y = 398.40;
    }
}

Answer 1

If you google about error 2007, you can see that this error is throwed by a displayobject container when you try to access a chil that dosen't exist in a parent container.

Use debug movie (ctrl+shift+enter) instead of test movie (ctrl+enter), like this you could see the line where the problem occurs and more info about the problem.

When you have your line just check the in the hitTestObject argument that the child you try to pass in exists, maybe it's just a typing error.