Vague error message after updating to React Router 7.7 and React 19.1

[GabenGar]

Reproduction

https://github.com/87906913/408919357/tree/main/react-router/vague-error-react-19

Steps are in the readme.

System Info

System:
    OS: Windows 10 
    CPU: (4)
    Memory: 7.80 GB
  Binaries:
    Node: 22.14.0
    npm: 10.8.0

Used Package Manager

npm

Expected Behavior

If it's something related to not supporting React 19, then the package should state so instead of throwing vague errors.

Actual Behavior

I presume it's some React error which was just re-thrown by React Router as is. Really fun to find out about this while working for another repro of another issue.

[timdorr]

You have mismatched and duplicate versions of React installed:

https://github.com/GabenGar/repros/blob/d5ec82953f8dc2dd71a1e4d3a8ce3663ee25e7f0/react-router/vague-error-react-19/package-lock.json#L4721-L4747
https://github.com/GabenGar/repros/blob/main/react-router/vague-error-react-19/package-lock.json#L1009-L1029

If you run npm update and also install sass-extended, it will resolve the issue.

[GabenGar]

That was the first time I see explicit usage of npm update. I always assumed it (or the procedure similar to) was ran automatically after npm install. I guess it was true back when plain npm install also modified the lockfile if it deemed necessary, and bring all sorts of reproducibility problems with it.
Where do you get sass-extended from? The message for me after fixing deps was installing sass-embedded.