After TCA places a node, Precise Maneuver no longer works

[alexandruast]

Ref: https://github.com/allista/ThrottleControlledAvionics/issues/64

Reproduce: Have Precise Maneuver plugin installed. Place a few maneuver nodes and confirm there are no errors. In orbit, run Orbital Autopilot -> Land. Abort the command, delete the node. Place a maneuver node again manually, Precise Maneuver won't come up, and it throws an error:

[EXC 07:27:18.486] NullReferenceException: Object reference not set to an instance of an object
	KSPPreciseManeuver.NodeTools.GetTargetOrbit (.CelestialBody refbody)
	KSPPreciseManeuver.NodeManager.UpdateNodes ()
	KSPPreciseManeuver.PreciseManeuver.Update ()