Error #1009 Simple Help please

Hello there guys, I been working on a small project on flash CS5.5 AS3, it is a side scrolling platform game.

However I been coming up with an error which is this :

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

at Corestructure_Scene1_fla::MainTimeline/movingPlayer()[Corestructure_Scene1_fla.MainTimeline::frame4:83]

And the line it points at is this one :

Velocity +=  gravity;

if (!floor.hitTestPoint(larry.x,larry.y,true))

{

larry.y +=  Velocity;

}

if (Velocity > 20)

{

Velocity = 20;

}

Now I dont know what to do, this error keep coming up as soon as i start the game, i move the character left and right, its fine, I press jump and the characters jumps fine, however, as soon as the character lands, this error keeps coming up and I just dont know how to fix it.

Any help would be great!