Fix for IE11 bug where document.activeElement sometimes returns a null-prototyped object.

Author: bicknellr

src/ShadowDOM/wrappers/Document.js

@@ -52,7 +52,10 @@
   defineWrapGetter(Document, 'head');
 
   defineGetter(Document, 'activeElement', function() {
-    var activeElement = wrap(unwrap(this).activeElement);
+    var unwrappedActiveElement = unwrap(this).activeElement;
+    if (!unwrappedActiveElement || !unwrappedActiveElement.nodeType) return null;
+
+    var activeElement = wrap(unwrappedActiveElement);
 
     // Loop while activeElement is not a shallow child of this document.
     while (!this.contains(activeElement)) {

src/ShadowDOM/wrappers/ShadowRoot.js

@@ -75,7 +75,10 @@
     },
 
     get activeElement() {
-      var activeElement = wrap(unwrap(this).ownerDocument.activeElement);
+      var unwrappedActiveElement = unwrap(this).ownerDocument.activeElement;
+      if (!unwrappedActiveElement || !unwrappedActiveElement.nodeType) return null;
+
+      var activeElement = wrap(unwrappedActiveElement);
 
       // Loop while activeElement is not a shallow child of this ShadowRoot.
       while (!this.contains(activeElement)) {