Link to the code that reproduces this issue

https://github.com/yehonatanyosefi/parallel-routes-example

To Reproduce

Happens randomly in production with App Router

Current vs. Expected behavior

Happens here: ../src/client/components/router-reducer/fill-lazy-items-till-leaf-with-head.ts in fillLazyItemsTillLeafWithHead at line 153:60:

const existingParallelRoutes = newCache.parallelRoutes.get(key)

Error: Cannot read properties of null (reading 'get')

Provide environment information

const nextConfig = {
  output: "standalone",
  reactStrictMode: true,
  poweredByHeader: false,
  skipTrailingSlashRedirect: true,
  ...
}

"next": "^15.0.0-rc.0",

### Which area(s) are affected? (Select all that apply)

Parallel & Intercepting Routes

### Which stage(s) are affected? (Select all that apply)

Other (Deployed)

### Additional context

_No response_