Safely retrieve router, improve page reload logic

[gurkerl83]

This PR improves the hot module reloading (HMR) logic in the dev client by:

• Safe Router Retrieval: Uses optional chaining (window.next?.router) to prevent errors if the router is uninitialized.
• Enhanced Error Page Handling: Ensures the client reloads when on /404 or /_error pages, addressing cases where page additions/removals can't be reliably matched to the current path.
• Improved Reload Logic: Checks if the current page is added or removed and triggers a reload accordingly to keep the client state up-to-date.
• Updated Comments: Clarifies the conditions under which reloads occur, especially when on error pages.

These changes make the HMR process more robust and prevent potential runtime issues.

Closes #71974

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 8c6bdf9

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[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
buildDuration	31s	24.3s	N/A
buildDurationCached	24s	24.4s	⚠️ +396ms
nodeModulesSize	416 MB	416 MB	⚠️ +7.68 kB
nextStartRea..uration (ms)	802ms	791ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
1187-HASH.js gzip	52.4 kB	52.4 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.36 kB	5.36 kB	N/A
bccd1874-HASH.js gzip	52.8 kB	52.8 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	231 B	235 B	N/A
main-HASH.js gzip	34.1 kB	34.1 kB	N/A
webpack-HASH.js gzip	1.71 kB	1.71 kB	N/A
Overall change	0 B	0 B	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Overall change	39.4 kB	39.4 kB	✓

Client Pages

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
_app-HASH.js gzip	193 B	193 B	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	512 B	510 B	N/A
css-HASH.js gzip	343 B	342 B	N/A
dynamic-HASH.js gzip	1.84 kB	1.84 kB	✓
edge-ssr-HASH.js gzip	265 B	265 B	✓
head-HASH.js gzip	363 B	362 B	N/A
hooks-HASH.js gzip	393 B	392 B	N/A
image-HASH.js gzip	4.49 kB	4.49 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.34 kB	N/A
routerDirect..HASH.js gzip	328 B	328 B	✓
script-HASH.js gzip	397 B	397 B	✓
withRouter-HASH.js gzip	323 B	326 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.59 kB	3.59 kB	✓

Client Build Manifests

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
_buildManifest.js gzip	749 B	746 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
index.html gzip	524 B	524 B	✓
link.html gzip	538 B	537 B	N/A
withRouter.html gzip	520 B	520 B	✓
Overall change	1.04 kB	1.04 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
edge-ssr.js gzip	128 kB	128 kB	N/A
page.js gzip	206 kB	206 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
middleware-b..fest.js gzip	670 B	666 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.2 kB	31.2 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
274-experime...dev.js gzip	322 B	322 B	✓
274.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	359 kB	359 kB	✓
app-page-exp..prod.js gzip	129 kB	129 kB	✓
app-page-tur..prod.js gzip	142 kB	142 kB	✓
app-page-tur..prod.js gzip	137 kB	137 kB	✓
app-page.run...dev.js gzip	348 kB	348 kB	✓
app-page.run..prod.js gzip	125 kB	125 kB	✓
app-route-ex...dev.js gzip	37.5 kB	37.5 kB	✓
app-route-ex..prod.js gzip	25.5 kB	25.5 kB	✓
app-route-tu..prod.js gzip	25.5 kB	25.5 kB	✓
app-route-tu..prod.js gzip	25.4 kB	25.4 kB	✓
app-route.ru...dev.js gzip	39.2 kB	39.2 kB	✓
app-route.ru..prod.js gzip	25.4 kB	25.4 kB	✓
pages-api-tu..prod.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru...dev.js gzip	11.6 kB	11.6 kB	✓
pages-api.ru..prod.js gzip	9.68 kB	9.68 kB	✓
pages-turbo...prod.js gzip	21.7 kB	21.7 kB	✓
pages.runtim...dev.js gzip	27.5 kB	27.5 kB	✓
pages.runtim..prod.js gzip	21.7 kB	21.7 kB	✓
server.runti..prod.js gzip	916 kB	916 kB	✓
Overall change	2.44 MB	2.44 MB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	gurkerl83/next.js Fix-HMR-Warning	Change
0.pack gzip	2.08 MB	2.08 MB	⚠️ +1.28 kB
index.pack gzip	74.8 kB	74.6 kB	N/A
Overall change	2.08 MB	2.08 MB	⚠️ +1.28 kB

Diff details

Diff for main-HASH.js

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Commit: b2c5188