java.lang.IllegalStateException: doAddContentChild(Object, int) did not add content child edu.illinois.mobius.atomic.advise.models.impl.AccessImpl@6802d023 (name: Access 1, codename: Access1, useDefault: true) (xpos: 517.0, ypos: 334.0, editMask: 0) at index 22, but at index 0.

@Override
	public final void addContentChild(Object contentChild, int index) {
		List<Object> oldContentChildren = new ArrayList<>(
				doGetContentChildren());
		if (oldContentChildren.contains(contentChild)) {
			int oldIndex = oldContentChildren.indexOf(contentChild);
			if (oldIndex == index) {
				throw new IllegalArgumentException("Cannot add " + contentChild
						+ " because its already contained at given index "
						+ index);
			} else {
				throw new IllegalArgumentException("Cannot add " + contentChild
						+ " because its already a content child at index "
						+ oldIndex);
			}
		}
		doAddContentChild(contentChild, index);
		// check doAddContentChild(Object, int) does not violate postconditions
		List<? extends Object> newContentChildren = doGetContentChildren();
		if (!newContentChildren.contains(contentChild)) {
			throw new IllegalStateException(
					"doAddContentChild(Object, int) did not add content child "
							+ contentChild + " .");
		}
		int newIndex = newContentChildren.indexOf(contentChild);
		if (newIndex != index) {
			throw new IllegalStateException(
					"doAddContentChild(Object, int) did not add content child "
							+ contentChild + " at index " + index
							+ ", but at index " + newIndex + ".");
		}
		contentChildren.setAll(newContentChildren);
	}

package edu.illinois.mobius.atomic.advise.ui.gef.parts;

import java.util.ArrayList;
import java.util.List;

import org.eclipse.gef.mvc.fx.parts.AbstractContentPart;
import org.eclipse.gef.mvc.fx.parts.IVisualPart;

import com.google.common.collect.HashMultimap;
import com.google.common.collect.SetMultimap;

import edu.illinois.mobius.atomic.advise.models.AEGNode;
import edu.illinois.mobius.atomic.advise.models.Arc;
import edu.illinois.mobius.atomic.advise.models.AttackExecutionGraph;
import javafx.scene.Group;
import javafx.scene.Node;

public class AttackExecutionGraphPart extends AbstractContentPart<Group> {
	
	private ArrayList<Object> children = new ArrayList<Object>();
	
	@Override
	protected void doAddChildVisual(IVisualPart<? extends Node> child, int index) {
		getVisual().getChildren().add(child.getVisual());
	}
	
	@Override
	protected void doAddContentChild(Object contentChild, int index) {
		if(contentChild instanceof AEGNode) {
			getContent().addNode((AEGNode)contentChild);
			children.add(index, contentChild);
		} else if(contentChild instanceof Arc) {
			getContent().addArc((Arc)contentChild);
			children.add(index, contentChild);
		} else {
            throw new IllegalArgumentException("contentChild has invalid type: " + contentChild.getClass());
        }
	}
	
	@Override
	protected Group doCreateVisual() {
		return new Group();
	}
	
	@Override
    protected SetMultimap<? extends Object, String> doGetContentAnchorages() {
        return HashMultimap.create();
    }

    @Override
    protected List<? extends Object> doGetContentChildren() {
    		for(Object o : getContent().getAllNodes())
    			if(!children.contains(o))
    				children.add(o);
    		for(Object o : getContent().getArcElements())
    			if(!children.contains(o))
    				children.add(o);
    		
    		return children;
    }

    @Override
    protected void doRefreshVisual(Group visual) {}

    @Override
    protected void doRemoveChildVisual(IVisualPart<? extends Node> child, int index) {
        getVisual().getChildren().remove(child.getVisual());
    }

    @Override
    protected void doRemoveContentChild(Object contentChild) {
        if (contentChild instanceof AEGNode) {
            getContent().removeNode((AEGNode)contentChild);
            children.remove(contentChild);
        } else if (contentChild instanceof Arc) {
        		getContent().removeArc((Arc)contentChild);
        		children.remove(contentChild);
        } else {
            throw new IllegalArgumentException("contentChild has invalid type: " + contentChild.getClass());
        }
    }
	
	@Override
	public AttackExecutionGraph getContent() {
		return (AttackExecutionGraph)super.getContent();
	}

}