Fix cached command builder

src/Connected.Data/Update/CommandBuilder.cs

@@ -28,6 +28,8 @@ internal abstract class CommandBuilder
 
 		if (TryGetExisting(out StorageOperation? existing))
 		{
+			if (existing is null)
+				return null;
 			/*
           * We need to rebuild an instance since StorageOperation
           * is immutable
@@ -43,7 +45,7 @@ internal abstract class CommandBuilder
 			if (result.Parameters is null)
 				return result;
 
-			foreach (var parameter in result.Parameters)
+			foreach (var parameter in existing.Parameters)
 			{
 				if (parameter.Direction == ParameterDirection.Input)
 				{