Fix runtime errors on develop

[ky28059]

Fixes:

• ClassCastException when attempting to use GRTNetworkTableEntry.getValue() -- it was returning a NetworkTableValue instance instead of the Object value
• NullPointerException in TurretSubsystem.periodic() after robot initialization -- this was an oversight with when jetson was non-null and it was attempting to calculate the hub location from previousPosition which was initialized to null

[e3l]

lgtm. We might get some weird behavior in the future with previousPosition when we reset the pose in auto..

[ky28059]

We might get some weird behavior in the future with previousPosition when we reset the pose in auto..

There's probably a more elegant solution in the auton sequential command groups to set turret's previous position at the end of the sequence (or even just when teleop initializes to call some turretSubsystem.resetPreviousPosition() method to set it back to current localization readings) but since auton is still a draft and we're always currently starting at (0, 0) this works for now