While obfuscation my app I got this issue from ProGuard:
Warning: org.apache.log4j.chainsaw.ControlPanel: can't find superclass or interface javax.swing.JPanel
This happens because log4j is using references to swing libraries that are not included in my library as reference.
To fix ProGuard error I added this line to ProGuard configuration:
Also, you can find a .cfg example on Android SDK version folder, like: $SDK_DIR/tools/proguard/proguard-android.txt.