Remove AllEventlisteners problem

Hi there,
I have recently started with flash and as3 (recently as in 3 days ago) and have run into a problem that I cannot solve. Was hoping someone can assist.

The source file is located here:

I am playing with this app as part of teaching myself Flash, so please be aware this is no creative exercise. If you run the file, you will notice that I have created a home screen (frame 1) and another called Shapes on frame 2.

If you navigate to shapes, you will see a couple of shapes along with a drag-able area (in green). This area selects a random object from the Array of shapes and allows you to only drop the shape selected for that turn. My issues is as this:

  1. If I navigate to shapes and straight back to home, I get this error:

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

and

  1. If, on the shapes screen, I move an object around (to any area on the stage) and then navigate back to the home screen, the object is still on the stage.

How do I remove these items and fix the error above?

The main question I had is how could I reload the shapes screen to allow to restart this game entirely?
So if on the shapes screen, you finished playing by successfully dragging the correct shape onto the square container, the game will reset with a new shape and allow you to play again?