React 19.1.0 "TypeError: Cannot read properties of null (reading 'useContext')"

[Evanion]

Reproduction

Evanion/RR7-issue

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (12) x64 13th Gen Intel(R) Core(TM) i7-1355U
    Memory: 14.79 GB / 31.64 GB
  Binaries:
    Node: 22.15.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.12.1 - ~\AppData\Roaming\npm\pnpm.CMD
  Browsers:
    Edge: Chromium (137.0.3296.52)
    Internet Explorer: 11.0.19041.5794
  npmPackages:
    @react-router/dev: ^7.6.3 => 7.6.3
    @react-router/node: ^7.6.3 => 7.6.3
    @react-router/serve: ^7.6.3 => 7.6.3
    react-router: ^7.6.3 => 7.6.3
    vite: ^6.0.0 => 6.3.5

Used Package Manager

npm

Expected Behavior

When running React-router, it should work with react 19.1.0

Actual Behavior

This only happens when running it in NX, I can't reproduce it in a plain react-router repo. But from what I know in how NX works, it's basically just an orchestration layer on top of the react-router cli.

When you upgrade from react 19.0.0 to 19.1.0, the site breaks. If you downgrade back to 19.0.0, it works again.
My though is that nx is probably only illuminating a flaky issue. as I have had the same issue in other plain react-router repos, but only on first render, and a reload would get you past it, but in this case it's persistent.

[timdorr]

Possibly related to #13991 or remix-run/remix#10455

[sergiodxa]

NX the monorepo tool? This error is most likely because you have two copies of RR or React.