EDIT

import java.util.List;

import java.util.stream.Collectors;

import org.eclipse.e4.ui.model.application.ui.basic.MPart;

import org.eclipse.e4.ui.workbench.modeling.EPartService;

import eu.etaxonomy.taxeditor.editor.definedterm.input.TermEditorInput;

import eu.etaxonomy.taxeditor.store.internal.TaxeditorStorePlugin;

		    	ITermTreeEditor sourceEditor = editor;

		    	if (droppedNode == null){

		            //the node does not come from the same editor, need to find the source editor

		            TermType type = ((TermNodeDto) droppedObject).getType();

		            TermEditorInput termEditorInput = new TermEditorInput(type);

		            EPartService partService = TaxeditorStorePlugin.getDefault().getWorkbench().getActiveWorkbenchWindow().getService(EPartService.class);

		            List<MPart> alreadyOpenedEditors = partService.getParts().stream()

		                    .filter(part->part.getObject()!=null && part.getObject() instanceof ITermTreeEditor)

		                    .filter(part->((ITermTreeEditor)part.getObject()).getTermType().equals(type))

		                    .collect(Collectors.toList());

		            if(!alreadyOpenedEditors.isEmpty()){

		                //there should never be more than one already opened editor

		                //so we just open the first

		                sourceEditor = (ITermTreeEditor)alreadyOpenedEditors.iterator().next().getObject();

		                droppedNode = sourceEditor.getNodeDtoForUuid(((TermNodeDto) droppedObject).getUuid());

		            }

		        }

		        if (droppedNode == null){

		            return false;

		        }

		    	TermNodeDto oldParent = sourceEditor.getNodeDtoForUuid(droppedNode.getParentUuid());