Java Nullpointer Exception in ArrayList (Lib GDX)

I have a problem with a "Nullpointer Exception" in LIb GDX.
When calling the method " public static ArrayList<BaseActor> getList(Stage stage, String className)"
I get the Nullpointer Exception.

Would be very grateful for help...

Here the function:

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

    Class theClass;
    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;
}

Here is the call to the function:

 public void update(float dt)
{

    try{
        for (BaseActor rockActor : BaseActor.getList(mainStage, "Rock"))
            turtle.preventOverlap(rockActor);
    }
    catch (Exception error)
    {  error.printStackTrace();  } ......

...

This is happening because the result of the try/catch could result in theClass still being null. One way to handle this would be to include your list logic inside of the try, like this:

try
{
    theClass = Class.forName(className);

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

return list;

And Chaosfire is right; you may need to do more than just print the stack trace.