How to make a linked waypoint editor

I'm working on making a editor that will add waypoints to a grid. I have the grid working and I can add waypoints to the grid no problem. What I am having trouble with is adding a link to the next waypoint that will be added to the grid. For example If I add a waypoint to the grid when no waypoints are added the last waypoint variable doesn't get filled, but if I add another waypoint the last waypoint variable will be filled with the waypoint before it.

The way my system is now I would have to add each link manually in the scene view. Is there a way to do this with a script so I don't have to manually do it by hand. (Basically add a gameobject to a public variable in a script in the scene view)