java.lang.ClassNotFoundException: Rock
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:264)
    at com.mygdx.game.BaseActor.getList(BaseActor.java:565)
    at com.mygdx.game.LevelScreen.update(LevelScreen.java:36)
    at com.mygdx.game.BaseScreen.render(BaseScreen.java:37)
    at com.badlogic.gdx.Game.render(Game.java:46)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:225)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:126
Exception in thread "LWJGL Application" java.lang.NullPointerException
    at com.mygdx.game.BaseActor.getList(BaseActor.java:571)
    at com.mygdx.game.LevelScreen.update(LevelScreen.java:36)
    at com.mygdx.game.BaseScreen.render(BaseScreen.java:37)
    at com.badlogic.gdx.Game.render(Game.java:46)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:225)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:126)

Process finished with exit code 0

public static ArrayList<BaseActor> getList(Stage stage, String className)
{
    ArrayList<BaseActor> list = new ArrayList<BaseActor>();

    Class theClass = null;
    try
    {  theClass = Class.forName(className);  }
    catch (Exception error)
    {  error.printStackTrace();  }

    for (Actor a : stage.getActors())
    {
        if ( theClass.isInstance( a ) )
            list.add( (BaseActor)a );
    }

    return list;
}