[fixed] appElement detection...

We can't use window.HTMLElement to check the `appElement`
when combined in a iframe.

window.HTMLElement !== window.HTMLElement (on the iframe)

src/helpers/safeHTMLElement.js

@@ -2,7 +2,18 @@ import ExecutionEnvironment from "exenv";
 
 const EE = ExecutionEnvironment;
 
-const SafeHTMLElement = EE.canUseDOM ? window.HTMLElement : {};
+const NodeTypeElement = 1;
+
+const IHTMLElement = function(props, propName) {
+  const element = props[propName];
+  return (
+    element.nodeType === NodeTypeElement &&
+    element.setAttribute &&
+    element.removeAttribute
+  );
+};
+
+const SafeHTMLElement = EE.canUseDOM ? IHTMLElement : {};
 
 export const SafeHTMLCollection = EE.canUseDOM ? window.HTMLCollection : {};