title: Cannot convert value of type 'String' to expected argument type 'SKNode'
body: <p>There is an issue on last line. How can I fix that? Issue says ''Cannot convert value of type 'String' to expected argument type 'SKNode'. '' Here is my code :</p>
  
  <pre><code>import SpriteKit
  
  let BallCategoryName = "ball"
  
  class GameScene: SKScene {
  
      let ball = childNodeWithName(BallCategoryName) as! SKSpriteNode
  </code></pre>