NullReferenceException in validation rule custom workflow

Hello community,

I am trying to check whether a contact reference on salesorder entity has its email address field filled or not.

After checking in the code, I went to salesorder entity tried to activate the record and then I got this exception:

Error in function ValidationUtil.activateSalesEntity.executeAction(): Unexpected exception from plug-in (Execute): Ewe.Xrm.Workflows.SalesEntityValidator.SalesEntityValidator: System.NullReferenceException: Object reference not set to an instance of an object.

My C# Function is as followed:

        private void ValidateIfAnsprechpartnerEmailAddressIsFilled()
        {
            var salesOrder = LocalContext.BaseEntity.BaseEntity;
            var ansprechpartnerRef = salesOrder?.GetAttributeValue<EntityReference>(SalesOrder.Properties.vis1_ansprechpartnerid);

            var ansprechpartnerMailAddress = LocalContext.Manager.Retrieve(ansprechpartnerRef, "emailaddress1");

            if (string.IsNullOrWhiteSpace(ansprechpartnerMailAddress.GetAttributeValue<string>("emailaddress1")))
            {
                ValidationsFailures.Add("Für den gewählten Ansprechpartner fehlt eine E-Mail-Adresse.");
            }
        }