Sim fails when returning pieces to bucket

[Denz1994]

Found during: #29

Steps to reproduce:
Note this is the most consistent means of reproduction, but it may not be limited to this interaction.

1. Open sim to Mixed-Numbers screen.
2. Click on level 1 (for low difficultly).
3. Solve a problem that involves using a whole shape.
4. Drag a new empty shape container into the play area.
5. Click on the plus sign to add more empty shape containers to the shape group within the play area.
6. Drag the whole shape pieces into the empty shape containers.
7. Click the minus button on the shape group.

Sorry for the long list of steps but I've finally been able to track down one method of reproducing this bug. I can demonstrate via video call if need be.

ShapeGroup.js?bust=1545250767622:133 Uncaught TypeError: this.returnPieceListener is not a function
at ShapeGroup.decreaseContainerCount (ShapeGroup.js?bust=1545250767622:133)
at Emitter.emit (Emitter.js?bust=1545250767622:202)
at PushButtonModel.fire (PushButtonModel.js?bust=1545250767622:155)
at Array.downPropertyObserver (PushButtonModel.js?bust=1545250767622:88)
at Emitter.emit3 (Emitter.js?bust=1545250767622:284)
at BooleanProperty._notifyListeners (Property.js?bust=1545250767622:288)
at BooleanProperty.setValueAndNotifyListeners (Property.js?bust=1545250767622:270)
at BooleanProperty.set (Property.js?bust=1545250767622:203)
at Emitter.emit3 (Emitter.js?bust=1545250767622:284)
at BooleanProperty._notifyListeners (Property.js?bust=1545250767622:288)

[jonathanolson]

I believe the above commit should solve this issue. Can you verify?

[Denz1994]

Works great. Closing this issue. Thanks.