Fixed NPE when AggregationTree is used for non-aggregations

pom.xml

@@ -5,7 +5,7 @@
         <modelVersion>4.0.0</modelVersion>
         <groupId>info.textgrid.services</groupId>
         <artifactId>aggregator</artifactId>
-        <version>1.4.4-SNAPSHOT</version>
+        <version>1.4.5-SNAPSHOT</version>
         <packaging>war</packaging>
         <name>TextGrid Aggregator Service</name>
 

src/main/java/info/textgrid/services/aggregator/tree/AggregationTreeFactory.java

@@ -93,7 +93,7 @@ protected boolean walk(final ObjectType object, final boolean again) {
 		else {
 			Aggregation parent = stack.peek();
 			lastEntry = new AggregationEntry(object, parent);
-			if (parent.getMetadata().getEdition() != null && 
+			if (parent != null && parent.getMetadata().getEdition() != null && 
 					refersTo(parent.getMetadata().getEdition().getIsEditionOf(), object))
 				parent.setWork(lastEntry);
 			return true;