add not null assertion

contribs/freight/src/test/java/org/matsim/freight/carriers/CarrierPlanXmlWriterV2_1Test.java

@@ -26,7 +26,6 @@
 import org.junit.jupiter.api.extension.RegisterExtension;
 import org.matsim.api.core.v01.Id;
 import org.matsim.api.core.v01.TransportMode;
-import org.matsim.freight.carriers.*;
 import org.matsim.freight.carriers.CarrierCapabilities.FleetSize;
 import org.matsim.testcases.MatsimTestUtils;
 import org.matsim.vehicles.Vehicle;
@@ -65,12 +64,15 @@ void test_whenReadingServices_nuOfServicesIsCorrect(){
 	void test_whenReadingCarrier_itReadsTypeIdsCorrectly(){
 
 		CarrierVehicle light = CarriersUtils.getCarrierVehicle(testCarrier, Id.createVehicleId("lightVehicle"));
+		assert light != null;
 		assertEquals("light",light.getVehicleTypeId().toString());
 
 		CarrierVehicle medium = CarriersUtils.getCarrierVehicle(testCarrier, Id.createVehicleId("mediumVehicle"));
+		assert medium != null;
 		assertEquals("medium",medium.getVehicleTypeId().toString());
 
 		CarrierVehicle heavy = CarriersUtils.getCarrierVehicle(testCarrier, Id.createVehicleId("heavyVehicle"));
+		assert heavy != null;
 		assertEquals("heavy",heavy.getVehicleTypeId().toString());
 	}