fix(react-dom): add null checks to completeSegment to prevent race condition

[ponharu]

Summary

This PR fixes a race condition bug in React DOM's Server-Side Rendering where the $RS (completeSegment) function doesn't check if parentNode is null before accessing it.

Problem:
During Suspense boundary resolution in production builds, the completeSegment function can encounter a race condition where DOM nodes are removed by other processes, causing parentNode to become null. This results in the error: Cannot read properties of null (reading 'parentNode').

Solution:
Added null safety checks for a, b, a.parentNode, and b.parentNode before DOM manipulation. This ensures the function safely exits if any required node or parent is missing.

Affected code:
packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js (line 15)

Fixes #35056

How did you test this change?

1. Minimal Reproduction:
Created a minimal Next.js application that consistently reproduces the bug:

• Repository: https://github.com/ponharu/react-issue
• Setup: Next.js 16.0.1 + React 19.2.0 + Chakra UI v3 with Suspense boundaries
• Steps: Production build with async page component (1s delay) + Suspense fallback

2. Verification:

• Before fix: Error occurred consistently when reloading the page multiple times
• After applying the fix: Error no longer occurs, page renders correctly
• Applied the fix to all affected compiled files in Next.js project and verified in production environment

3. Commands run:

# Applied fix to production build
npm run build
npm run start

# Tested multiple page reloads to trigger race condition
# Monitored browser console for errors

4. Environment tested:

• Next.js 16.0.1 (production build)
• React/React DOM 19.2.0
• Chakra UI v3
• Node.js 24.11.0
• Browser: google Chrome 142.0.7444.60

Note: This is a minimal defensive fix that adds null checks without changing the core logic. The race condition is timing-sensitive and more likely to occur with CSS-in-JS libraries like Chakra UI.

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