Fix picking points outside of volume

[errollgarner]

• When handling LandmarkRegistrationObject points outside of the main volume, no point would get selected in the widget after a capture and a subsequent capture would crash the app. This behaviour is undesired. While it may be rare to handle LandmarkRegistrationObject points not associated with a loaded volume, the case can still arise.
• Current solution: Validation that points are within the bounds of the volume aren't enforced anymore and in the event that the user tries to capture when no point is selected, we simply move on without capturing anything and display an error message.