I had the crash problem, and I also confirm that this is the right fix -- update the two sets of two values, and you should be good to go.
For future reference for newbies to come, you'll need to find where your mods are kept (in Windows, it's C:\Users\%USERNAME%\AppData\Roaming\Factorio\mods\WellPlanner_0.18.7.zip -- AppData is a hidden folder, so you may need to get cute with view settings to make it visible; in other OSes, YMMV, Google is your friend). Unzip the file, click into the unzipped mod folder, edit control.lua, and go to the lines outlined above, make the changes Aerukath recommends, save, and close. Then, re-zip the folder as the same name and overwrite the "stock" version of WellPlanner in your mod directory. If it works, then you should be able to open Factorio, load your save, and right-click on a created WellPlanner object without a crash.
It's a hack and not the "right" way to update a mod (you should use Git and update your versions and metadata at least in the "happy path"!), but this is a Q&D fix until the author can publish a 0.18.8 to fix the bug.
... Of course, if there's a better way to do a Q&D patch, I'm always happy to learn a better way! :-)