EDIT

      <command

            defaultHandler="eu.etaxonomy.taxeditor.editor.view.derivate.handler.CreateFieldUnitHandler"

            id="eu.etaxonomy.taxeditor.editor.derivate.createFieldUnit"

            name="%command.name.43">

      </command>

package eu.etaxonomy.taxeditor.editor.view.derivate.handler;

import org.eclipse.core.commands.AbstractHandler;

import org.eclipse.core.commands.ExecutionEvent;

import org.eclipse.core.commands.ExecutionException;

import eu.etaxonomy.cdm.api.service.IOccurrenceService;

import eu.etaxonomy.cdm.model.common.CdmBase;

import eu.etaxonomy.cdm.model.occurrence.FieldUnit;

import eu.etaxonomy.taxeditor.editor.view.derivate.DerivateView;

import eu.etaxonomy.taxeditor.model.AbstractUtility;

import eu.etaxonomy.taxeditor.model.MessagingUtils;

import eu.etaxonomy.taxeditor.store.CdmStore;

public class CreateFieldUnitHandler extends AbstractHandler {

    @Override

    public Object execute(ExecutionEvent event) throws ExecutionException {

        if(AbstractUtility.getActivePart() instanceof DerivateView){

            DerivateView derivateView = (DerivateView)AbstractUtility.getActivePart();

            if(derivateView.isDirty()){

                MessagingUtils.warningDialog(DerivateView.VIEW_HAS_UNSAVED_CHANGES, this, DerivateView.YOU_NEED_TO_SAVE_BEFORE_PERFORMING_THIS_ACTION);

                return null;

            }

            FieldUnit fieldUnit = FieldUnit.NewInstance();

            fieldUnit = CdmBase.deproxy(CdmStore.getService(IOccurrenceService.class).save(fieldUnit), FieldUnit.class);

            derivateView.getConversationHolder().commit();

            derivateView.addFieldUnit(fieldUnit);

            derivateView.refreshTree();

        }

        return null;

    }

}

package eu.etaxonomy.taxeditor.handler;

import org.eclipse.core.commands.AbstractHandler;

import org.eclipse.core.commands.ExecutionEvent;

import org.eclipse.core.commands.ExecutionException;

import org.eclipse.jface.viewers.ISelection;

import org.eclipse.jface.viewers.IStructuredSelection;

import org.eclipse.ui.handlers.HandlerUtil;

public class CreateFieldUnitHandler extends AbstractHandler {

    @Override

    public Object execute(ExecutionEvent event) throws ExecutionException {

        ISelection currentSelection = HandlerUtil.getCurrentSelection(event);

        if(currentSelection instanceof IStructuredSelection){

            System.out.println(currentSelection);

        }

        return null;

    }

}