I can't open the demo project with my Godot 4.3 editor. The demo and the entire code is made with Godot 4.4, so it shouldn't be compatible.
Heads up for potential buyers.
Viewing post in Grid Building Plugin for Godot 4 comments
I have it working on 4.3 stable. I will reupload the demo just encase there is some small difference that's causing you problems. Did you open with 4.3 after getting the warnings about 4.4? I have been editing in 4.4 but I don't believe I added any specific code that should not be 4.3 compatible like typed dictionaries.
Edit: I reuploaded Grid Building v4.2.0 ~ All Packages (Requires Godot 4.3+)

I get these error messages with both Godot 4.3 and 4.4 (fresh downloads each)
https://imgur.com/a/IuRgnhZ