NullPointerException at a musicplayer

im creating a musicplayer for School, but im getting a NullPointerException on the following Code, i have absolutly no idea why. I think the Problem is caused by Title t = ... new Artist(...)

 Player initialized
java.lang.NullPointerException
    at de.hofuniversity.soundmaster.data.Playlist.fromDirectory(Playlist.java:168)
    at de.hofuniversity.soundmaster.Main.start(Main.java:35)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Unknown Source)

from Direcotry:

public static Playlist fromDirectory(File dir) 
{
    Playlist p = new Playlist();
    File[] files = dir.listFiles(new FilenameFilter()
    {   
        @Override       
        public boolean accept(File dir, String name)
        {
            return name.toLowerCase().endsWith(".mp3");
        }
    });


    for(File f : files)
    {

        try 
        {
            AudioFile mp3 = AudioFileIO.read(f);
            Title t = new Title(mp3.getTag().getFirstTitle(), new Artist(mp3.getTag().getFirstArtist()), mp3.getLength()*1000);
            t.setFile(f);
            p.add(t);

        } 
        catch (Exception e) 
        {
            e.printStackTrace();
        }

    }
    return p;
}

Artist

    public class Artist 
{
    private String surname, givenname;
    private long birthdate;



    public Artist(String firstArtist) 
    {
    }
    public String getSurname() 
    {
        return surname;
    }

    public void setSurname(String surname) 
    {
        this.surname = surname;
    }

    public String getGivenname() 
    {
        return givenname;
    }

    public void setGivenname(String givenname) 
    {
        this.givenname = givenname;
    }

    public long getBirthdate() 
    {
        return birthdate;
    }

    public void setBirthDate(long birthdate) 
    {
        this.birthdate = birthdate;
    }

    @Override
    public String toString() 
    {
        return "Artist [surname=" + surname + ", givenname=" + givenname + "]";
    }
}