TypeError: Error #1009: Cannot access a property or method of a null object reference. at RECOVER_fyp1_fla::MainTimeline/abc1()

Hi, what does this mean? Can anybody figure it out? Thanks.

Code:

swatter.addEventListener(Event.ENTER_FRAME,abc1);

Mouse.hide();



function abc1(e:Event)
{
    swatter.x = mouseX;
    swatter.y = mouseY;

    enter code here

    mosq1.y = mosq1.y + 2;
    mosq2.y = mosq2.y + 3;
    mosq3.y = mosq3.y + 4;
    mosq4.y = mosq4.y + 5;
    mosq5.y = mosq5.y + 6;



    if (mosq1.y > 640)
    {
        mosq1.y = -50;
    }

    if (mosq2.y > 640)
    {
        mosq2.y = -50;
    }

    if (mosq3.y > 640)
    {
        mosq3.y = -50;
    }

    if (mosq4.y > 640)
    {
        mosq4.y = -50;
    }

    if (mosq5.y > 640)
    {
        mosq5.y = -50;
    }


    if(swatter.hitTestObject(mosq1))
    {
        //SoundMixer.stopAll();
        //three_start_sound1.play();
        mosq1.parent.removeChild(mosq1);
    }

    if(swatter.hitTestObject(mosq2))
    {
        //SoundMixer.stopAll();
        //three_start_sound1.play();
        mosq2.parent.removeChild(mosq2);
    }

    if(swatter.hitTestObject(mosq3))
    {
        //SoundMixer.stopAll();
        //three_start_sound1.play();
        mosq3.parent.removeChild(mosq3);
    }

    if(swatter.hitTestObject(mosq4))
    {
        //SoundMixer.stopAll();
        //three_start_sound1.play();
        mosq4.parent.removeChild(mosq4);
    }

    if(swatter.hitTestObject(mosq5))
    {
        //SoundMixer.stopAll();
        //three_start_sound1.play();
        mosq5.parent.removeChild(mosq5);
    }
}

enter code here