Handle more explicitly a potential error case

Author: fredericDelaporte

src/NHibernate/Action/EntityInsertAction.cs

@@ -49,10 +49,12 @@ public override void Execute()
 			// else inserted the same pk first, the insert would fail
 			if (!veto)
 			{
-				if (id is DelayedPostInsertIdentifier delayed && delayed.ActualId != null)
+				if (id is DelayedPostInsertIdentifier delayed)
 				{
 					wasDelayed = true;
-					id = delayed.ActualId;
+					id = delayed.ActualId ??
+						throw new InvalidOperationException(
+							$"The delayed foreign identifier {delayed} has not been resolved before insertion of a {instance}");
 				}
 				persister.Insert(id, State, instance, Session);
 

src/NHibernate/Async/Action/EntityInsertAction.cs

@@ -45,10 +45,12 @@ public override async Task ExecuteAsync(CancellationToken cancellationToken)
 			// else inserted the same pk first, the insert would fail
 			if (!veto)
 			{
-				if (id is DelayedPostInsertIdentifier delayed && delayed.ActualId != null)
+				if (id is DelayedPostInsertIdentifier delayed)
 				{
 					wasDelayed = true;
-					id = delayed.ActualId;
+					id = delayed.ActualId ??
+						throw new InvalidOperationException(
+							$"The delayed foreign identifier {delayed} has not been resolved before insertion of a {instance}");
 				}
 				await (persister.InsertAsync(id, State, instance, Session, cancellationToken)).ConfigureAwait(false);