title: LibGdx app Generate Signed APK got error
body: <p>I made a libgdx app and trying to build with Android studio and Proguard. The problem is that I can run it well in debug mode. But when I generate APK file, run it, it has this error :</p>
  
  <pre><code>Error reading file: ui/uiskin.json
              at com.badlogic.gdx.utils.Json.a(Unknown Source)
              at com.badlogic.gdx.scenes.scene2d.ui.Skin.a(Unknown Source)
              at com.badlogic.gdx.scenes.scene2d.ui.Skin.&lt;init&gt;(Unknown Source)
              at jangkoo.a.a.a.a(Unknown Source)
              at jangkoo.a.e.af.a(Unknown Source)
              at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(Unknown Source)
              at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1520)
              at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1248)
       Caused by: com.badlogic.gdx.utils.SerializationException: Field not found: down (com.badlogic.gdx.scenes.scene2d.ui.Button$ButtonStyle)
  </code></pre>
  
  <p>Clearly the last line says Field not found: down in json file.
  What's wrong? what should I do?</p>