[fix] remove invariant error from Portal

src/components/Portal/index.js

@@ -6,7 +6,6 @@
  * @flow
  */
 
-import invariant from 'fbjs/lib/invariant'
 import Platform from '../../apis/Platform'
 import React, { Component, PropTypes } from 'react'
 import StyleSheet from '../../apis/StyleSheet'
@@ -81,10 +80,6 @@ class Portal extends Component {
   }
 
   render() {
-    invariant(
-      _portalRef === this || _portalRef === undefined,
-      'More than one Portal instance detected. Never use <Portal> in your code.'
-    )
     _portalRef = this
     if (!this.state.modals) { return null }
     const modals = []