fix(ui): safe call within useEffect teardown (#11135)

`NavProvider` useEffects teardown is trying to set `style` on an element
that may not exist. The original code produces the following error:


![image](https://github.com/user-attachments/assets/11a83fbe-67eb-42a9-bd78-749ea98b67c5)

![image](https://github.com/user-attachments/assets/28ed2534-2387-416b-8191-d68b478161aa)

Therefore, a condition has been added to check if `navRef.current` is
truthy.

packages/ui/src/elements/Nav/context.tsx

@@ -96,7 +96,9 @@ export const NavProvider: React.FC<{
   // when the component unmounts, clear all body scroll locks
   useEffect(() => {
     return () => {
-      navRef.current.style.overscrollBehavior = 'auto'
+      if (navRef.current) {
+        navRef.current.style.overscrollBehavior = 'auto'
+      }
     }
   }, [])