Merge branch 'dev' into 'master'

Version 3.15.0

See merge request !8

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/check/error/PolygonWithoutSurfaceError.java

@@ -10,11 +10,14 @@ import de.hft.stuttgart.citydoctor2.check.ModificationListener;
 import de.hft.stuttgart.citydoctor2.datastructure.GmlElement;
 import de.hft.stuttgart.citydoctor2.datastructure.Polygon;
 
+import java.io.Serial;
+
 public class PolygonWithoutSurfaceError implements CheckError {
 
+	@Serial
 	private static final long serialVersionUID = 2676579487514583105L;
 	
-	private Polygon p;
+	private final Polygon p;
 
 
 	public PolygonWithoutSurfaceError(Polygon polygon) {