Sensor shows arrow with 0 V/m

[BryceAG]

For phetsims/qa#37
Test device: Hanson

Operating System: Win 10

Browser: Chrome Version 61.0.3163.79 (Official Build) (64-bit)

Problem description: If a sensor is placed right in-between two identical charges, it will correctly show the value of 0 V/m, but the arrow displayed will remain the arrow from before it read 0 v/m. This is done most easily with snap to grid so it snaps into place between the two charges.

Steps to reproduce:
Turn on Values
Turn on grid
Turn on snap to grid
Place two identical charges on the screen spaced apart so that the midpoint can have something snap into its place
Place a sensor on the midpoint

Screenshot:

This was found in phet-io, where the report a problem option was unavailable, so there is no more troubleshooting information

[BryceAG]

Also the degrees value says 0 regardless of what position it is pointing, and in State the upper and lower sims will point in different directions, with the lower sim always pointing in the same direction regardless of the direction the upper sim is pointing.

[samreid]

I am the developer helping bring this sim to PhET-iO, but it would be good if @jonathanolson has time to help work on sim bugs like this.