为什么此 Linq 方法会引发 Null 引用异常

问：

我有一个我的控制器继承自。它重写该方法。该方法将从 中检索 JWT 令牌，并使用它来检索发出请求的用户。BaseApiControllerInitializeHttpControllerContext

    public class BaseApiController : ApiController
    {
        public static tUser CurrentUser;
        public BaseApiController()
        {

        }

        protected override void Initialize(HttpControllerContext controllerContext)
        {
            base.Initialize(controllerContext);

            var request = controllerContext.Request;
            if (request.Headers.Authorization != null && request.Headers.Authorization.Scheme.Equals("bearer", StringComparison.OrdinalIgnoreCase))
            {
                CurrentUser = Helpers.JwtAuthentication.UserToken(request.Headers.Authorization.Parameter);
            }
        }
    }    

调用该方法时，会间歇性地发生错误。这是方法。UserToken

    public static tUser UserToken(string token)
    {
        string username = ExtractUserName(token);

        if (string.IsNullOrEmpty(username))
            return null;

        try
        {
            tUser user = Repository.DB.tUsers.Where(u => u.UserName == username && u.IsDeleted == false).FirstOrDefault();
            return user;
        }
        catch (Exception ex)
        {
            return null;
        }
    }

异常被抛在了线上，我不知道为什么。如果我检查代码行中的各种对象，它们不是 null。如果我再次在代码行上执行调试器，它运行不会有问题。tUser user = Repository.DB.tUsers.Where(u => u.UserName == username && u.IsDeleted == false).FirstOrDefault();

为什么这行代码会间歇性地抛出错误“对象引用未发送到对象的实例”？

    public class Repository
    {
        public static Entities DB = new Entities(ConfigurationManager.AppSettings["ConnectionString"].ToString());
    }

    public partial class Entities : DbContext
    {
        public Entities(string secret) : base(Helpers.KeyVault.GetSecret(secret))
        {
            this.Configuration.LazyLoadingEnabled = false;
        }
    }