[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)

Author: diasbruno

src/components/Modal.js

@@ -57,7 +57,7 @@ class Modal extends Component {
       })
     ]),
     appElement: PropTypes.oneOfType([
-      PropTypes.instanceOf(SafeHTMLElement),
+      SafeHTMLElement,
       PropTypes.instanceOf(SafeHTMLCollection),
       PropTypes.instanceOf(SafeNodeList),
       PropTypes.arrayOf(PropTypes.instanceOf(SafeHTMLElement))

src/helpers/safeHTMLElement.js

@@ -2,7 +2,20 @@ 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];
+  const is = element.nodeType === NodeTypeElement &&
+	element.setAttribute &&
+	element.removeAttribute;
+
+  if (!is) {
+    return new Error(`Warning: Failed %s type: %s%s', 'prop', Invalid prop \`${propName}\` supplied to \`Modal\``); 
+  }
+};
+
+export const SafeHTMLElement = EE.canUseDOM ? IHTMLElement : {};
 
 export const SafeHTMLCollection = EE.canUseDOM ? window.HTMLCollection : {};