[fixed] Safety check for SSR (reactjs#668)

- add missing test for string appElement
- add safety check for using setElement on ssr
- fix minor typo

specs/Modal.spec.js

@@ -84,6 +84,28 @@ export default () => {
     ReactDOM.unmountComponentAtNode(node);
   });
 
+  it("allow setting appElement of type string", () => {
+    const node = document.createElement("div");
+    class App extends Component {
+      render() {
+        return (
+          <div>
+            <Modal isOpen>
+              <span>hello</span>
+            </Modal>
+          </div>
+        );
+      }
+    }
+    const appElement = "body";
+    Modal.setAppElement(appElement);
+    ReactDOM.render(<App />, node);
+    document.body
+      .querySelector(".ReactModalPortal")
+      .parentNode.should.be.eql(document.body);
+    ReactDOM.unmountComponentAtNode(node);
+  });
+
   it("default parentSelector should be document.body.", () => {
     const modal = renderModal({ isOpen: true });
     modal.props.parentSelector().should.be.eql(document.body);

src/helpers/ariaAppHider.js

@@ -1,4 +1,5 @@
 import warning from "warning";
+import { canUseDOM } from "./safeHTMLElement";
 
 let globalElement = null;
 
@@ -12,7 +13,7 @@ export function assertNodeList(nodeList, selector) {
 
 export function setElement(element) {
   let useElement = element;
-  if (typeof useElement === "string") {
+  if (typeof useElement === "string" && canUseDOM) {
     const el = document.querySelectorAll(useElement);
     assertNodeList(el, useElement);
     useElement = "length" in el ? el[0] : el;