Not initialized fields create a NullPointerException on save

import java.io.File;
import java.util.HashSet;

import net.cubespace.Yamler.Config.Config;

public class TestConfig extends Config
{
    public HashSet<String> data;

    public TestConfig()
    {
        CONFIG_FILE = new File("test.yml");
        try
        {
            init();
        } catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    public static void main(String args[])
    {
        new TestConfig();
    }
}

This creates the following NullPointerException:

net.cubespace.Yamler.Config.InvalidConfigurationException: Could not save the Field
    at net.cubespace.Yamler.Config.Config.internalSave(Config.java:84)
    at net.cubespace.Yamler.Config.Config.save(Config.java:36)
    at net.cubespace.Yamler.Config.Config.init(Config.java:108)
    at TestConfig.<init>(TestConfig.java:15)
    at TestConfig.main(TestConfig.java:24)
Caused by: java.lang.NullPointerException
    at java.util.AbstractCollection.addAll(AbstractCollection.java:343)
    at net.cubespace.Yamler.Config.Converter.Set.fromConfig(Set.java:54)
    at net.cubespace.Yamler.Config.InternalConverter.fromConfig(InternalConverter.java:85)
    at net.cubespace.Yamler.Config.Config.internalSave(Config.java:81)
    ... 4 more

Cube-Space / Yamler

Not initialized fields create a NullPointerException on save #20