LibGdx app Generate Signed APK got error

Question

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 :

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.<init>(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)

Clearly the last line says Field not found: down in json file. What's wrong? what should I do?