1.12.2 Error getting yaml files list name

I have this code

Code (Text):

public class ClasePrincipal extends JavaPlugin implements Listener{
   
    private File file = null;
    private FileConfiguration fc = null;
   
    SetupPlugin sp = new SetupPlugin();
    @Override
    public void onEnable() {
        Bukkit.getConsoleSender().sendMessage("skere");
        registerConfig();
        Bukkit.getPluginCommand("menu").setExecutor(new Command(this));
        sp.plugin(this);
        registerConfig();
        registerCustomConfig();
    }
    public void registerConfig() {
        File config = new File(this.getDataFolder(),"config.yml");
        if(!config.exists()){
            this.getConfig().options().copyDefaults(true);
            saveConfig();
        }
    }
    public void saveYML() {
        try {
            fc.save(file);
            }catch(IOException e) {
            e.printStackTrace();
        }
    }
    public String registerNewConfig() {
        File[] files = folder().listFiles();
        for(int i = 0; i < files.length; i++) {
            if(files[i].isFile()) {
                return files[i].getName();      
            }
        }
        return registerNewConfig();
    }
    public File folder() {
        File folder = new File(this.getDataFolder() + File.separator + "Inventory");
        if(!folder.exists()) {
            folder.mkdir();
        }
        return folder;
    }
    public FileConfiguration getFiles() {
        if(fc == null) {
            reloadCustomConfig();
        }
        return fc;
    }
    public void reloadCustomConfig() {
        if(file != null)  {
            file = new File(folder(), registerNewConfig());
        }
       
        Reader defConfigStream;  
       
        fc = YamlConfiguration.loadConfiguration(file);
            try {
                defConfigStream = new InputStreamReader(this.getResource(registerNewConfig()), "UTF8");

               
                if (defConfigStream != null) {
                    YamlConfiguration defConfig = YamlConfiguration.loadConfiguration(defConfigStream);

                    fc.setDefaults(defConfig);
                }
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }
    }
    public void registerCustomConfig() {
        File file2 = new File(this.getDataFolder() + File.separator + "Inventory", "ejemplo.yml");
        FileConfiguration fc2 = YamlConfiguration.loadConfiguration(file2);
        file = new File(folder(), registerNewConfig());
        if(!file.exists() ) {
            this.getFiles().options().copyDefaults(true);
            saveYML();
        }
        if(!file2.exists()) {
            fc2.options().copyDefaults(true);
            try {
                fc2.save(file2);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
   
}
 

What it does is get the names of all the files in a directory (this.getDataFolder + File.Separator + "Inventory") turns out to work to a point, when there are no files inside the Inventory folder it throws this error:

Code (Text):

java.lang.StackOverflowError: null
        at com.theunclebarto.mein.ClasePrincipal.folder(ClasePrincipal.java:54) ~[?:?]
        at com.theunclebarto.mein.ClasePrincipal.registerNewConfig(ClasePrincipal.java:45) ~[?:?]
        at com.theunclebarto.mein.ClasePrincipal.registerNewConfig(ClasePrincipal.java:51) ~[?:?]
        at com.theunclebarto.mein.ClasePrincipal.registerNewConfig(ClasePrincipal.java:51) ~[?:?]
        at com.theunclebarto.mein.ClasePrincipal.registerNewConfig(ClasePrincipal.java:51) ~[?:?]
        at com.theunclebarto.mein.ClasePrincipal.registerNewConfig(ClasePrincipal.java:51) ~[?:?]
and repeat too many times "at com.theunclebarto.mein.ClasePrincipal.registerNewConfig (Main Class.java:51) ~ [?:?]"
 

Can you help me?

~ Thanks in the advance