title: image set has an unassigned child
body: <p>I've been working on an app in Xcode for a few months now. I came back to work on it after not having looked at it for a while and suddenly, I had 174 issues.</p>
  <p>All of my images on the storyboard were replaced with question marks, with the error:</p>
  <blockquote>
  <p>The image set __ has unassigned children</p>
  </blockquote>
  <p>In <code>Assets.xcassets</code> all of the images are labelled as unassigned:</p>
  <p><img src="https://i.stack.imgur.com/nKF6a.png" alt="An example" /></p>
  <p>I don't know what to make of this, as it was all working fine a few weeks ago.</p>