title: react native ui kitten button icon
body: <p>in react-native ui kitten Button when i want to make a condition about loading to show the Spinner icon it not work when the icon props return null or false</p>
  
  <p><a href="https://i.stack.imgur.com/n7mi5.png" rel="nofollow noreferrer">Image of code</a></p>
  
  <p>when the icon exist it shows like this so its good for me
  <a href="https://i.stack.imgur.com/RIpMB.png" rel="nofollow noreferrer">image of app response</a></p>
  
  <p>but when the response is false i gives a error i hope its not a bug :'(</p>
  
  <p><a href="https://i.stack.imgur.com/qZWR9.png" rel="nofollow noreferrer">Image of error</a></p>