This is what my test code looks like:

var formIdentity = input as FormIdentity;
var formContainerBlock = formIdentity.GetFormBlock();
var webhookActorModels = formContainerBlock.Content.Property.GetPropertyValue<IEnumerable<WebhookActorModel>>("CallWebhookAfterSubmissionActor").ToList();
if (webhookActorModels == null || webhookActorModels.Count() > 0)
{
    webhookActorModels[0].BaseUrl = "google.com";
}
formContainerBlock.Content.SetPropertyValue("CallWebhookAfterSubmissionActor", webhookActorModels);

And the error I get: