Access java builder pattern with JNI from Delphi

Question

I'm currently working on an implementation of a jar file in delphi via the JNI. Here's my problem: I'm unable to create an object via the java implemented builder.

Here's the java code:

public class MyObject {
   // .. code
   public static class Builder {
       protected MyObject instance;

       public Builder() {
          this.instance = new MyObject
       }

       public Builder withString(String s) {
          instance.string = s;
          return this;
       }

       public MyObject Build() {
          // check object values
          return instance;
       }
   }
}

As you can see, the builder class is a subclass of the object.

In delphi, I use this code to access the builder:

jClass := JNIEnv.FindClass('package/MyObject$Builder');
jMid := JNIEnv.GetMethodID(jClass, '<init>', '()V');
jObj := JNIEnv.NewObjectV(jClass, jMid, '');

_jMid := JNIEnv.GetMethodID(jClass, 'withString', '(Ljava/lang/String;)package/MyObject$Builder;');
_jObj := JNIEnv.CallObjectMethodV(jObj, _jMid, 'string');

But I can't seem to get this working, I always get an access violation.

Thanks in advance!