Can confirm that switching wall types out several times quickly will still cause a crash. In this case it wasn't quite while the previous wall was despawning, but on like the third wall type to be placed on a tile within a few seconds.

(My testing area being those 5 tiles in the middle of the green wall there. The crash seemed to happen while placing back down green again, after being a blue tile, purple tile and purple gate. But that could be either coincidence, or a slight delay between cause and effect, or anything.)