Double click on the error message (or right click and “Find in tree”). This will lead you to the “response” in the containment tree. Then use “Goto/Usage in Diagram”. If “None” is displayed then this parameter node is not used in your activity diagram – it is a duplicate (probably by copying it). Use right click at the “response” object from the activity diagram and you will see the correct node. (see image “usageInDiagram.png).