Fix bug report with-portals example vercel#5694

examples/with-portals/components/Portal.js

@@ -4,13 +4,10 @@ import ReactDOM from 'react-dom'
 export class Portal extends React.Component {
   componentDidMount () {
     this.element = document.querySelector(this.props.selector)
+    this.forceUpdate()
   }
 
   render () {
-    if (this.element === undefined) {
-      return null
-    }
-
     return ReactDOM.createPortal(this.props.children, this.element)
   }
 }