Bug: Race condition in completeSegment ($RS) causes "Cannot read properties of null (reading 'parentNode')" error

The $RS (completeSegment) function in React DOM's Server-Side Rendering lacks null safety checks on parentNode, causing a race condition error when Suspense boundaries resolve in production builds.

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

Root Cause: File: packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js (line 15)

The completeSegment function doesn't check if a.parentNode or b.parentNode is null before accessing it. During Suspense resolution, nodes can be removed by other processes, setting parentNode to null.

Buggy Code: https://github.com/facebook/react/blob/v19.2.0/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js#L15

export const completeSegment =
  '$RS=function(a,b){a=document.getElementById(a);b=document.getElementById(b);for(a.parentNode.removeChild(a);a.firstChild;)b.parentNode.insertBefore(a.firstChild,b);b.parentNode.removeChild(b)};';

Proposed Fix:

export const completeSegment =
  '$RS=function(a,b){a=document.getElementById(a);b=document.getElementById(b);if(a&&b&&a.parentNode&&b.parentNode){for(a.parentNode.removeChild(a);a.firstChild;)b.parentNode.insertBefore(a.firstChild,b);b.parentNode.removeChild(b)}};';

React version: 19.2.0

Steps To Reproduce

1. Clone the reproduction repository
2. Run npm install
3. Run npm run build (production build required)
4. Run npm run start
5. Navigate to http://localhost:3000
6. Note: The bug may not occur on the first page load. Reload the page several times if needed.
7. Check browser console for the error