title: spread doesn't work with null
body: <p>ES6 spread won't work this way</p>
  
  <pre><code>I have this.state = { sortedKey: null } in my constructor and I do
  
  this.setState({
  sortedKey: [...this.state.sortedKey, 'something']
  })
  </code></pre>
  
  <p>I got error of <code>Cannot convert undefined or null to object</code>?</p>