Looks like you are being stopped from even entering the prefab world due to a messed up file.
Look in youruserdirectory > AppData > Romaing > 7DaysToDie > Saves > Worlds > Playtesting
If you have previously loaded prefabs and saved, you will have worlds in here.
When there is an update, these worlds can have problems just like regular saves.
Try deleting all of the folders in there, if any.
(Each of those worlds has a main.ttw file by the way. Problem is I have no idea which one has the bad one. In my opinion, the prefab editor should not save a world, just the prefab. I don't know why they do this. When I launch the prefab editor, I don't really care if it auto-loads the last prefab. I would much rather the world was always empty until I load a prefab. This way, when there is an update, I don't have to worry about these saved worlds too)
Now, if you don't have any worlds saved in there, then the main.ttw it is referring to is the one it grabs from the other 7D2D folder (found wherever your steam apps are saved) If that is the case, delete main.ttw from SteamApps > 7 Days To Die >Data > Worlds > Playtesting and possibly SteamApps > 7 Days To Die >Data > Worlds > Empty ... and then revalidate so fresh copies of these are installed.