added null check for subject

Author: pernillehofgaard

src/Altinn.App.Core/Implementation/PrefillSI.cs

@@ -224,6 +224,16 @@ when await systemUser.LoadDetails() is { } details && details.Party.PartyId == p
                 {
                     var datasetName = dataset.SelectToken("name")?.ToString();
                     var subject = !string.IsNullOrWhiteSpace(party.SSN) ? party.SSN : party.OrgNumber;
+
+                    if (string.IsNullOrEmpty(subject))
+                    {
+                        _logger.LogError(
+                            "Could not prefill from {DanKey}, no valid subject (SSN or OrgNumber) found for party",
+                            _danKey
+                        );
+                        continue;
+                    }
+
                     var fields = dataset.SelectToken("mappings");
                     if (fields != null)
                     {