@PostConstruct
public void createControls(Composite parent){
   parent.setLayout(new GridLayout(1, false));
   //more interface stuff
   //call the referencemodel
   final IReferenceModelSchema model = AModelService.getInstance(); //<<-- Here the Exception occurs
   for(Entry<String, SimpleSchemaItem> ssi:openehr.getRootMap().entrySet()){
      treeItem  = new TreeItem (previousItem, SWT.NONE);
      treeItem.setText(ssi.getKey());
   }
}