fixing null pointer when clearing a vertex adjacency list

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/datastructure/Vertex.java

@@ -132,6 +132,6 @@ public class Vertex extends Vector3d {
 	 * Remove all adjacent rings from this vertex, ignoring geometry association
 	 */
 	void clearAdjacentRings() {
-		adjacentRings = null;
+		adjacentRings = new ArrayList<>(2);
 	}
 }