Cannot switch from Planet X to Custom gravity

[arouinfar]

It is currently impossible to switch from Planet X to Custom gravity. When using the drop-down menu to switch from an option with a known value of gravity to Custom, the slider maintains its current position/value. However, when switching from Planet X to Custom, the value of g should return to whatever value it had immediately before Planet X was selected.

The desired behavior is best exemplified in the Bending Light Java sim, so using it as an example here. (http://phet.colorado.edu/en/simulation/bending-light)

Setting a custom value: Using the drop-down menu to select an option with an unknown value: Using the drop-down menu to return to the custom option:

Another example: Selecting water from the drop-down menu: Choosing an option with an unknown value: Selecting custom:

Assigning to @andrey-zelenkov

[andrey-zelenkov]

Fixed switching from Planet X to Custom Planet. Reassign to @arouinfar for verification.

[arouinfar]

Looks good! Closing.