The fallback to finding ninja in PATH doesn't work on non-Windows systems. The current implementation searches for a component in PATH that ends with "Ninja" or "ninja". This works fine on Windows, if for example C:\Program Files\Ninja has been added to PATH. The most likely situation on Linux is that ninja is located in /usr/bin/ninja and /usr/bin is in PATH. This means the PATH component doesn't match and ninja won't be found.

Also the error message is cryptic. "> No valid path to Ninja found. From path: null/ninja." Improvement here is the search returns null?