Fix: Add BoundarySurfaces list initialization

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

@@ -12,13 +12,14 @@ import org.xmlobjects.gml.model.geometry.primitives.Solid;
 import org.xmlobjects.gml.model.geometry.primitives.SolidProperty;
 
 import java.io.Serial;
+import java.util.ArrayList;
 import java.util.List;
 
 public class CityFurniture extends CityObject implements BoundedSpace{
 
     @Serial
     private static final long serialVersionUID = 3325661061543962473L;
-    private List<BoundarySurface> boundarySurfaces;
+    private List<BoundarySurface> boundarySurfaces = new ArrayList<>();
     private org.citygml4j.core.model.cityfurniture.CityFurniture cgmlCityFurniture;