[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/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/components/ModalPortal.js

@@ -47,7 +47,7 @@ export default class ModalPortal extends Component {
     htmlOpenClassName: PropTypes.string,
     ariaHideApp: PropTypes.bool,
     appElement: PropTypes.oneOfType([
-      PropTypes.instanceOf(SafeHTMLElement),
+      SafeHTMLElement,
       PropTypes.instanceOf(SafeHTMLCollection),
       PropTypes.instanceOf(SafeNodeList),
       PropTypes.arrayOf(PropTypes.instanceOf(SafeHTMLElement))

src/helpers/safeHTMLElement.js

@@ -2,7 +2,23 @@ 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];
+
+  if (!element || element.nodeType !== NodeTypeElement) return;
+
+  const isValid = Boolean(element.setAttribute && element.removeAttribute);
+
+  if (!isValid) {
+    return new Error(
+      `Warning: Invalid prop \`${propName}\` supplied to \`Modal\``
+    );
+  }
+};
+
+export const SafeHTMLElement = EE.canUseDOM ? IHTMLElement : {};
 
 export const SafeHTMLCollection = EE.canUseDOM ? window.HTMLCollection : {};