The issue that you are seeing is because you are trying to instantiate a new ContactRepository().

The appropriate way to reference the repository is by creating the object through Sitecore's Configuration system.

private ContactRepository _repository = Sitecore.Configuration.Factory.CreateObject("tracking/contactRepository", true) as ContactRepository;

The issue that you are seeing is because you are trying to instantiate a new ContactRepository().

The appropriate way to reference the repository is by creating the object through Sitecore's Configuration Factory.

private ContactRepository _repository = Sitecore.Configuration.Factory.CreateObject("tracking/contactRepository", true) as ContactRepository;