Unexpected Error when writing a blank list of complex fences

I wanted to clear out all complex fences from my Rover (running master) so I did this:

1. connected with MP over wifi (I used the ESP wifi radio)
2. On the Plan page:
a. selected the Fence drop-down from the top right
b. pushed the Read button and about 12-ish lines appeared below
c. deleted all rows with the X in the Delete column
d. pushed the Write button. At this point I got the "unexpected error" message. When I pushed the Read button the rows did not re-appear so it appears that it did what I wanted but the error message will make users worry

Reproduce the issue in Mission Planner's Plan page using the Fence drop-down: read the existing complex fences, delete every row, and write the blank list. Trace the write operation from the Fence controls and verify that an empty list is saved without showing an unexpected error when read again.

Written by the indexing model from the issue text.