[fixed] check if the modal content is available when async update... (r…

…eactjs#1)

It seems that the using redux, we can get a async update in which can fail
when trying to focus the content of the modal.

This should be a temporary fix since the issue is hard to reproduce,
but safe for other applications.

src/components/ModalPortal.js

@@ -130,7 +130,8 @@ export default class ModalPortal extends Component {
   }
 
   // Don't steal focus from inner elements
-  focusContent = () => (!this.contentHasFocus()) && this.content.focus();
+  focusContent = () => 
+    (this.content && !this.contentHasFocus()) && this.content.focus();
 
   closeWithTimeout = () => {
     const closesAt = Date.now() + this.props.closeTimeoutMS;