Call non-static method (from subclass) in Java from Unity

Hello!

I know there are tons of threads here regarding this topic... I've read them all... I have been trying this for 4 hours straight and my nerves are raw right now.

Here is what I want to do:

I have a class myProductSettings extended from Activity in Java which contains another class PrefsFragment extended from PreferenceFragment. In this class (PrefsFragment) I want to call a non-static method from Unity to send over some preferences (CharSequence[] eventually).

I've tried it in many different ways but I am always getting "JNI: Init'd AndroidJavaObject with null ptr!". And later "JNI: Unable to find method id for 'ReceiveCallFromUnity'".

The static call to myProductSettings works fine so far .... But I was not able to call any non-static method.

How can I get a reference to the instance of PrefsFragment (mPrefsFragment) and call a non-static function on it from Unity?

Any help is highly appreciated!!! Thanks!

Here is my setup in Java:

 public class myProductSettings extends Activity  {
     
     public static PrefsFragment mPrefsFragment;
      
     @Override
     protected void onCreate(Bundle savedInstanceState) {
                 super.onCreate(savedInstanceState);       
 
                 // Display the fragment as the main content.
                 FragmentManager mFragmentManager = getFragmentManager();
                 FragmentTransaction mFragmentTransaction = mFragmentManager
                                         .beginTransaction();
                 mPrefsFragment = new PrefsFragment();
                 mFragmentTransaction.replace(android.R.id.content, mPrefsFragment);
                 mFragmentTransaction.commit();
                
      
     }

 public static void testMessage() {
         
         Log.i(myProduct.TAG, "testMessage called from Unity to myProductSettings");
         
     }
     
 
     public class PrefsFragment extends PreferenceFragment 
     {

         
     
         @Override
         public void onCreate(Bundle savedInstanceState) {
             super.onCreate(savedInstanceState);

 [......]
             
         }
         
         
         public void ReceiveSceneNamesFromUnity(CharSequence[] sceneNames) {
             
             Log.i(myProduct.TAG, sceneNames[0].toString());
             
         }
         
         public void ReceiveCallFromUnity() {
             
             Log.i(myProduct.TAG, "Non-Static call from Unity to PrefsFragment");
             
         }
     
     }
 }

And here my setup in Unity (C#)

     void Start () {
         
         StartCoroutine("TestSend");
     }
     
     IEnumerator TestSend(){
         
         yield return new WaitForSeconds(5);
         
         SendSceneNamesToJava();
     
     }
         
     private void SendSceneNamesToJava() {
         
 #if UNITY_ANDROID && !UNITY_EDITOR
         
         AndroidJavaClass jc;
         AndroidJavaObject currentActivity, myProductSettings, jo;
         
         jc = new AndroidJavaClass("com.test.test.myProductSettings");
           jo = jc.GetStatic<AndroidJavaObject>("mPrefsFragment");
         
         jo.Call("ReceiveCallFromUnity");
         
         
 #endif

 }