[fixed] check if focusLaterElements is empty before popping

Without a check to see if the focusLaterElements array is empty, we
end up calling `pop()` on an empty array—resulting in an undefined
value—and inadvertently jump to the `catch` block when we attempt to
call `focus()` on `undefined`.

This ensures that if there are no existing elements to return focus to
once our modal is closed (this happens often in a testing env), we do
not throw a TypeError by accident.

Author: Austin Wood

Diff for: src/helpers/focusManager.js

@@ -37,8 +37,10 @@ export function markForFocusLater() {
 export function returnFocus() {
   let toFocus = null;
   try {
-    toFocus = focusLaterElements.pop();
-    toFocus.focus();
+    if (focusLaterElements.length !== 0) {
+      toFocus = focusLaterElements.pop();
+      toFocus.focus();
+    }
     return;
   } catch (e) {
     console.warn(