conditional reference to HTMLIFrameElement

This will prevent an exception being thrown in workers.

src/uv.handler.js

@@ -700,7 +700,10 @@ function __uvHook(window, config = {}, bare = '/bare/') {
         },
     });
 
-    const contentWindowGet = Object.getOwnPropertyDescriptor(
+    // HTMLIFrameElement may not be defined (workers)
+    const contentWindowGet =
+        HTMLIFrameElement &&
+        Object.getOwnPropertyDescriptor(
             HTMLIFrameElement.prototype,
             'contentWindow'
         ).get;