EDIT

      <command

            defaultHandler="eu.etaxonomy.taxeditor.editor.handler.OpenDerivateViewHandler"

            id="eu.etaxonomy.taxeditor.editor.openSpecimenEditor"

            name="Open Specimen Editor">

         <commandParameter

               id="eu.etaxonomy.taxeditor.specimenUuidParameter"

               name="Specimen UUID parameter"

               optional="true"

               typeId="eu.etaxonomy.taxeditor.specimenUuidParameterType">

         </commandParameter>

      </command>

   <extension

         point="org.eclipse.ui.commands">

      <commandParameterType

            converter="eu.etaxonomy.taxeditor.handler.UuidParameterConverter"

            id="eu.etaxonomy.taxeditor.specimenUuidParameterType"

            type="java.util.UUID">

      </commandParameterType>

   </extension>

        Set<UUID> selectedElementUUIDs = new HashSet<UUID>();

        //check if parameter is set

        Object parameter = event.getObjectParameterForExecution("eu.etaxonomy.taxeditor.specimenUuidParameter");

        if(parameter instanceof UUID){

            selectedElementUUIDs.add((UUID) parameter);

        }

        else{

            //if not, try with current selection

            ISelection currentSelection = HandlerUtil.getCurrentSelection(event);

            if(currentSelection instanceof IStructuredSelection){

                Iterator<?> selectionIterator = ((IStructuredSelection) currentSelection).iterator();

                while(selectionIterator.hasNext()){

                    Object object = selectionIterator.next();

                    if(object instanceof SpecimenOrObservationBase<?>){

                        selectedElementUUIDs.add(((SpecimenOrObservationBase<?>) object).getUuid());

                    }

                    else if(object instanceof IndividualsAssociation){

                        SpecimenOrObservationBase specimen = ((IndividualsAssociation) object).getAssociatedSpecimenOrObservation();

                        if(specimen!=null){

                            selectedElementUUIDs.add(specimen.getUuid());

                        }

        }

        if(!selectedElementUUIDs.isEmpty()){

            DerivateViewEditorInput input = new DerivateViewEditorInput(selectedElementUUIDs);

            try {

                EditorUtil.open(input);

            } catch (PartInitException e) {

                MessagingUtils.error(OpenDerivateViewHandler.class, "Could not open Derivative Editor", e);

            } catch (NullPointerException npe){

                MessagingUtils.messageDialog("Failed to open Editor", OpenDerivateViewHandler.class, "Could not open Derivative Editor. The derivate hierarchy is corrupted!", npe);

        else{

            MessagingUtils.informationDialog("Empty selection", "No Specimen selected.");

        }

package eu.etaxonomy.taxeditor.handler;

import java.util.UUID;

import org.eclipse.core.commands.AbstractParameterValueConverter;

import org.eclipse.core.commands.ParameterValueConversionException;

public class UuidParameterConverter extends AbstractParameterValueConverter {

    public UuidParameterConverter() {

        // TODO Auto-generated constructor stub

    }

    @Override

    public Object convertToObject(String parameterValue) throws ParameterValueConversionException {

        return UUID.fromString(parameterValue);

    }

    @Override

    public String convertToString(Object parameterValue) throws ParameterValueConversionException {

        return parameterValue.toString();

    }

}

import java.util.HashMap;

import java.util.Map;

import java.util.UUID;

import org.eclipse.core.commands.Command;

import org.eclipse.core.commands.ExecutionException;

import org.eclipse.core.commands.NotEnabledException;

import org.eclipse.core.commands.NotHandledException;

import org.eclipse.core.commands.ParameterizedCommand;

import org.eclipse.core.commands.common.NotDefinedException;

import org.eclipse.ui.IWorkbenchWindow;

import org.eclipse.ui.PlatformUI;

import org.eclipse.ui.commands.ICommandService;

import org.eclipse.ui.handlers.IHandlerService;

import eu.etaxonomy.taxeditor.model.AbstractUtility;

import eu.etaxonomy.taxeditor.model.MessagingUtils;

import eu.etaxonomy.taxeditor.view.derivateSearch.DerivateSearchCompositeController;

	/**

	 * {@inheritDoc}

	 */

	@Override

	public boolean performFinish() {

	    boolean performFinish = super.performFinish();

        IHandlerService handlerService = (IHandlerService) AbstractUtility.getService(IHandlerService.class);

        //get the command from plugin.xml

        IWorkbenchWindow window = PlatformUI.getWorkbench().getActiveWorkbenchWindow();

        ICommandService cmdService = (ICommandService)window.getService(ICommandService.class);

        Command command = cmdService.getCommand("eu.etaxonomy.taxeditor.editor.openSpecimenEditor");

        Map<String, UUID> parameters = new HashMap<String, UUID>();

        parameters.put("eu.etaxonomy.taxeditor.specimenUuidParameter", getEntity().getUuid());

        ParameterizedCommand parameterizedCommand = ParameterizedCommand.generateCommand(command, parameters);

        //execute the command

        try {

            handlerService.executeCommand(parameterizedCommand, null);

        } catch (ExecutionException e) {

            MessagingUtils.error(DerivateSearchCompositeController.class, e);

        } catch (NotDefinedException e) {

            MessagingUtils.error(DerivateSearchCompositeController.class, e);

        } catch (NotEnabledException e) {

            MessagingUtils.error(DerivateSearchCompositeController.class, e);

        } catch (NotHandledException e) {

            MessagingUtils.error(DerivateSearchCompositeController.class, e);

        }

        return performFinish;

	}