[chore] Return null for portal when modal is closed

Author: Ives van Hoorne

Diff for: specs/Modal.spec.js

@@ -30,6 +30,11 @@ describe('State', () => {
     expect(ReactDOM.findDOMNode(mcontent(modal))).toNotExist();
   });
 
+  it('doesn\'t render the portal if modal is closed', () => {
+    const modal = renderModal({}, 'hello');
+    expect(ReactDOM.findDOMNode(modal.portal)).toNotExist();
+  })
+
   it('has default props', () => {
     const node = document.createElement('div');
     Modal.setAppElement(document.createElement('div'));

Diff for: src/components/ModalPortal.js

@@ -265,7 +265,7 @@ export default class ModalPortal extends Component {
     const contentStyles = className ? {} : defaultStyles.content;
     const overlayStyles = overlayClassName ? {} : defaultStyles.overlay;
 
-    return this.shouldBeClosed() ? <div /> : (
+    return this.shouldBeClosed() ? null : (
       <div
         ref={this.setOverlayRef}
         className={this.buildClassName('overlay', overlayClassName)}