Avoid TypeError in PageLayout checking document.body (#5552)

* Avoid TypeError in PageLayout checking document.body

* Make eslint happy

Author: camertron

.changeset/lazy-jars-leave.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": patch
+---
+
+Avoid accessing properties of potentially null document.body in PageLayout

packages/react/src/PageLayout/PageLayout.tsx

@@ -364,21 +364,24 @@ const VerticalDivider: React.FC<React.PropsWithChildren<DividerProps & Draggable
       window.addEventListener('keydown', handleKeyDrag)
       window.addEventListener('mouseup', handleDragEnd)
       window.addEventListener('keyup', handleKeyDragEnd)
-      document.body.setAttribute('data-page-layout-dragging', 'true')
+      const body = document.body as HTMLElement | undefined
+      body?.setAttribute('data-page-layout-dragging', 'true')
     } else {
       window.removeEventListener('mousemove', handleDrag)
       window.removeEventListener('mouseup', handleDragEnd)
       window.removeEventListener('keydown', handleKeyDrag)
       window.removeEventListener('keyup', handleKeyDragEnd)
-      document.body.removeAttribute('data-page-layout-dragging')
+      const body = document.body as HTMLElement | undefined
+      body?.removeAttribute('data-page-layout-dragging')
     }
 
     return () => {
       window.removeEventListener('mousemove', handleDrag)
       window.removeEventListener('mouseup', handleDragEnd)
       window.removeEventListener('keydown', handleKeyDrag)
       window.removeEventListener('keyup', handleKeyDragEnd)
-      document.body.removeAttribute('data-page-layout-dragging')
+      const body = document.body as HTMLElement | undefined
+      body?.removeAttribute('data-page-layout-dragging')
     }
   }, [isDragging, isKeyboardDrag, currentWidth, minWidth, maxWidth])