Index: Ringtoets/HeightStructures/src/Ringtoets.HeightStructures.Service/HeightStructuresCalculationService.cs
===================================================================
diff -u -rebdbf01f6ab35a28b3a306145796ce532920af62 -r4febca1a6186a1c6a09aa673ce967ff0b7efd0b1
--- Ringtoets/HeightStructures/src/Ringtoets.HeightStructures.Service/HeightStructuresCalculationService.cs	(.../HeightStructuresCalculationService.cs)	(revision ebdbf01f6ab35a28b3a306145796ce532920af62)
+++ Ringtoets/HeightStructures/src/Ringtoets.HeightStructures.Service/HeightStructuresCalculationService.cs	(.../HeightStructuresCalculationService.cs)	(revision 4febca1a6186a1c6a09aa673ce967ff0b7efd0b1)
@@ -73,7 +73,10 @@
         /// </summary>
         public void Cancel()
         {
-            calculator?.Cancel();
+            if (calculator != null)
+            {
+                calculator.Cancel();
+            }
             canceled = true;
         }