SpeedControllerGroup looses children on reorder

[AustinShalit]

When moving a SpeedControllerGroup around in the tree it looses some of its children. It looks like it has a 50% retention rate.

Steps to reproduce:

1. Open a RobotBuilder project
2. Create a Subsystem
3. Add a RobotDrive to the created subsystem
4. Add a speed controller group to the subsystem
5. Add speed controllers to the speed controller group
6. Move the speed controllers to the RobotDrive

This also works when moving the speed controller group from the RobotDrive to another subsystem or subsystem to subsystem.

[sciencewhiz]

@SamCarlberg I've seen this also, would you have time to look at it? The speed controllers are still in the map (can't add another device that uses the same port), but missing from the GUI.