title: Closure cannot implicitly capture a mutating self parameter
body: <p>I am using Firebase to observe event and then setting an image inside completion handler</p>
  
  <pre><code>FirebaseRef.observeSingleEvent(of: .value, with: { (snapshot) in
          if let _ = snapshot.value as? NSNull {
              self.img = UIImage(named:"Some-image")!
          } else {
              self.img = UIImage(named: "some-other-image")!
          }
  })
  </code></pre>
  
  <p>However I am getting this error</p>
  
  <blockquote>
    <p>Closure cannot implicitly capture a mutating self parameter</p>
  </blockquote>
  
  <p>I am not sure what this error is about and searching for solutions hasn't helped</p>