Fixup null reference access in Breadcrumbs computeVisibleItems. (#2146)

* Fixup null reference access in Breadcrumbs `computeVisibleItems`.

TL;DR the `current` field on a ref is `T | null` not `T` and we must
guard against invalid access by first interrogating `current`.

Since refs are initialized as null and can become null at runtime, blind
access to `current` can lead `TypeError`s (as cited in #1979).

In practice it seems this _can happen reliably_ in situations where a
`Breadcrumb` component is being unmounted as part of a larger component
tree.

Unfortunately, I have not been able to reduce this error case to a small
reproduction, therefore I have not been able to add a test explicitly
to signal regressions.

Still, and this is quite troubling, the Typescript compiler should be
flagging invalid access such as this. I'm not sure if something has been
disarmed in the tsconfig to assume `T | null` is `T` in this case, but
a similar access in a [TS playground](https://www.typescriptlang.org/play?ts=3.8.3#code/JYWwDg9gTgLgBAJQKYEMDG8BmUIjgcilQ3wG4AoczAVwDsNgJbElMBJWzJI+pACgCUcAN7k4cMXDRMAzvCKY4AXhboYAOmoykyTAB4AEgBUAsgBkAqmeByAogBskIJLRgA+PrWr37AiuMkAekCWTBkpFGYAIyQ4Lx84FHk6GFAkdUlpWjkpAAtgewATHmU4BXU0aigeDTR8op4KSSIYKuZ4+woAX3IgA)
is flagged as an error.

This aims to fix #1979.

* Rephrase comment about why we're coercing the type inference.

Author: onelson

packages/@react-spectrum/breadcrumbs/src/Breadcrumbs.tsx

@@ -56,8 +56,14 @@ function Breadcrumbs<T>(props: SpectrumBreadcrumbsProps<T>, ref: DOMRef) {
 
   let updateOverflow = useCallback(() => {
     let computeVisibleItems = (visibleItems: number) => {
-      let listItems = Array.from(listRef.current.children) as HTMLLIElement[];
-      let containerWidth = listRef.current.offsetWidth;
+      // Refs can be null at runtime.
+      let currListRef: HTMLUListElement | null = listRef.current;
+      if (!currListRef) {
+        return;
+      }
+
+      let listItems = Array.from(currListRef.children) as HTMLLIElement[];
+      let containerWidth = currListRef.offsetWidth;
       let isShowingMenu = childArray.length > visibleItems;
       let calculatedWidth = 0;
       let newVisibleItems = 0;