Why is [Owin] throwing a null exception on new project?

Similar to Sandeep’s answer, I also updated the cookie authentication provider. Except, instead of swallowing the error I threw the exception so you could see what the underlying problem was:

app.UseCookieAuthentication(new CookieAuthenticationOptions
{
    AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
    LoginPath = new PathString("/Account/Login"),
    Provider = new CookieAuthenticationProvider
    {
        // Enables the application to validate the security stamp when the user logs in.
        // This is a security feature which is used when you change a password or add an external login to your account.  
        OnValidateIdentity = SecurityStampValidator.OnValidateIdentity<ApplicationUserManager, User>(
            validateInterval: TimeSpan.FromMinutes(30),
            regenerateIdentity: (manager, user) => user.GenerateUserIdentityAsync(manager)),

            /* I changed this part */
            OnException = (context =>
            {
                throw context.Exception;
            })
    }                
});

The underlying problem for me was that I had changed the model and forgotten to add a new migration.