Add filter to skip the modifying when the zone of space is null (#3525)

Co-authored-by: Jinxuan He <jinxuan.he@rhdhv.com>
Co-authored-by: Dimitrie Stefanescu <didimitrie@gmail.com>
Co-authored-by: Claire Kuang <kuang.claire@gmail.com>

Objects/Converters/ConverterRevit/ConverterRevitShared/PartialClasses/ConvertZone.cs

@@ -146,7 +146,7 @@ public partial class ConverterRevit
     {
       revitZone = ConvertZoneToRevit(speckleSpace.zone, out _);
     }
-    else if (revitZone != null && revitZone.Phase.Name != targetPhase.Name)
+    else if (revitZone != null && revitZone.Phase.Name != targetPhase.Name && speckleSpace.zone != null)
     {
       Doc.Delete(revitZone.Id);