conditional reference to HTMLIFrameElement

This will prevent an exception being thrown in workers.

src/uv.handler.js

@@ -700,10 +700,13 @@ function __uvHook(window, config = {}, bare = '/bare/') {
         },
     });
 
-    const contentWindowGet = Object.getOwnPropertyDescriptor(
+    // HTMLIFrameElement may not be defined (workers)
-        HTMLIFrameElement.prototype,
+    const contentWindowGet =
-        'contentWindow'
+        HTMLIFrameElement &&
-    ).get;
+        Object.getOwnPropertyDescriptor(
+            HTMLIFrameElement.prototype,
+            'contentWindow'
+        ).get;
 
     function uvInject(that) {
         const win = contentWindowGet.call(that);