SmartDashboard can't display DifferentialDrive or MecanumDrive in LiveWindow mode.

[KeithRieck]

The new RobotDriveBase classes are automatically being added to LiveWindow mode. However, these elements cause SmartDashboard to experience a NullPointerException whenever we switch into Test mode.

Subsystem "Ungrouped" does not contain widget "robotDrive"
Table: NetworkTable: /LiveWindow/Ungrouped/robotDrive
Type: DifferentialDrive
Trying to add a widget of type "null" and key robotDrive
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at edu.wpi.first.smartdashboard.livewindow.elements.LWSubsystem.addSubsystemElement(LWSubsystem.java:92)
        at edu.wpi.first.smartdashboard.livewindow.elements.LWSubsystem.access$000(LWSubsystem.java:31)
        at edu.wpi.first.smartdashboard.livewindow.elements.LWSubsystem$1$1.run(LWSubsystem.java:133)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
        at java.awt.EventQueue.access$500(EventQueue.java:97)
        at java.awt.EventQueue$3.run(EventQueue.java:709)
        at java.awt.EventQueue$3.run(EventQueue.java:703)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)