Uncaught Error: Error calling method on NPObject in javascript

I want to use an applet to change a repository. To do this I wrote the following code:

LoadOntology:

package owlapi.loader;

import org.semanticweb.owlapi.apibinding.OWLManager;
import org.semanticweb.owlapi.io.*;
import org.semanticweb.owlapi.model.*;
import org.semanticweb.owlapi.util.DefaultPrefixManager;
import org.semanticweb.owlapi.util.OWLEntityRenamer;

import java.io.File;
import java.util.Collections;
import java.util.List;
import java.util.Map;


public class LoadOntology {
    private static String             ontologyDir   = "./owlapi/loader/featurepool.owl";
    private static OWLOntology        localOntology;
    private static OWLOntologyManager manager;
    private static OWLDataFactory     factory;
    private static PrefixManager      pm            = new DefaultPrefixManager("http://wise.vub.ac.be/Members/lamia/variability/Feature_Assembly/FAM.owl#");

public LoadOntology() {
    manager = OWLManager.createOWLOntologyManager();
    File file = new File(ontologyDir);
    try {
        localOntology = manager.loadOntologyFromOntologyDocument(file);
        System.out.println("Loaded ontology: " + localOntology);
        factory = manager.getOWLDataFactory();
        }   
    catch (UnparsableOntologyException e) {
        System.out.println("Could not parse the ontology: " + e.getMessage());
        Map<OWLParser, OWLParserException> exceptions = e.getExceptions();
        for (OWLParser parser : exceptions.keySet()) {
            System.out.println("Tried to parse the ontology with the " + parser.getClass().getSimpleName() + " parser");
            System.out.println("Failed because: " + exceptions.get(parser).getMessage());
        }
    }
    catch (UnloadableImportException e) {
        System.out.println("Could not load import: " + e.getImportsDeclaration());
        OWLOntologyCreationException cause = e.getOntologyCreationException();
        System.out.println("Reason: " + cause.getMessage());
    }
    catch (OWLOntologyCreationException e) {
        System.out.println("Could not load ontology: " + e.getMessage());
    }   
}

public void addInstance(String Class, String individual) {
    OWLClass owlClass = factory.getOWLClass(Class, pm);
    OWLClass superClass = factory.getOWLThing();
    OWLIndividual instance = factory.getOWLNamedIndividual(individual, pm);

    OWLClassAssertionAxiom classAssertion1 = factory.getOWLClassAssertionAxiom(superClass, instance);
    manager.addAxiom(localOntology, classAssertion1);
    OWLClassAssertionAxiom classAssertion2 = factory.getOWLClassAssertionAxiom(owlClass, instance);
    manager.addAxiom(localOntology, classAssertion2);
    System.out.println("Instance added");
}
}

InJava (applet loaded in javascript):

public class InJava extends Applet{
private static LoadOntology loadedOntology;

public void addInstance(String Class, String individual) {
    Graphics g = getGraphics();
     g.drawString("Test", 10, 10);
    loadedOntology = new LoadOntology();
    loadedOntology.addInstance(Class, individual);
    }
}

When I load the applet, there is no error at all, but when I try to call the method addInstance:

<input type="button" value="call Java Applet method" onClick = 'document.owlapi.addInstance("Abstract_Feature", "New_Test")'>
<APPLET CODE="owlapi/InJava.class" NAME="owlapi" HEIGHT=100 WIDTH=100> </APPLET>  

The browser throws the error Uncaught Error: Error calling method on NPObject. After reading some questions about this on stackoverflow, I came only to the conclusion that there has to be something wrong with the plugin.

I added some testing-code to the method (the drawString("Test", 10, 10)) which works, so I suggest that the constructing of the LoadOntology class gives some issues (this class was tested separately in works perfect on its own). Does anybody see what I am doing wrong?

update: I added the a part of the java class that is used by the applet: the repository loader. All the provided code should compile.

update: I didn't use MASCRIPT/SCRIPTABLE because the code is based on this example, which works in my current browser (chrome)

udpate: You have to include the owlapi jars to the buildpath, which can be downloaded here