wrong title for subgraph clusters

src/main/java/kieker/analysis/graph/export/dot/DotElementExporter.java

@@ -29,7 +29,7 @@ class DotElementExporter extends AbstractTransformer<Void> {
 			if (vertex.hasChildGraph()) {
 				Graph childGraph = vertex.getChildGraph();
 
-				dotGraphWriter.addClusterStart(childGraph.getName());
+				dotGraphWriter.addClusterStart(vertex.getId().toString());
 
 				Map<String, String> attributes = transformProperties(vertex);
 				attributes.putAll(transformProperties(childGraph));