[Alan Anderson]

A reference won't be in the registry until you put it there. That's what happens in the Begin vi. Open the sensor and use a Refnum Set to give it a name.

Are you using the rangefinder from the Kit of Parts? The most straightforward way to read it is using an Analog channel. I think the Ultrasonic Get Range function you're using is for the SRF04 part, a different style of ultrasonic sensor. Look at the examples for the MB1010 instead.