Weird NullReference Exception on registering User with extra information gathering

Question

I have an unhandled exception, which relates to adding a dropdown which if i dont bind it the register page has an empty dropdown. On registering a user i need to choose a role for that user.

 public async Task<IActionResult> Register(RegisterViewModel model, string returnUrl = null)
    {
        ViewData["ReturnUrl"] = returnUrl;
        if (ModelState.IsValid)
        {
            var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
            var result = await _userManager.CreateAsync(user, model.Password);
            if (result.Succeeded)
            {
                _logger.LogInformation("User created a new account with password.");

                var code = await _userManager.GenerateEmailConfirmationTokenAsync(user);
                var callbackUrl = Url.EmailConfirmationLink(user.Id, code, Request.Scheme);
                await _emailSender.SendEmailConfirmationAsync(model.Email, callbackUrl);

                await _signInManager.SignInAsync(user, isPersistent: false);
                _logger.LogInformation("User created a new account with password.");
                return RedirectToLocal(returnUrl);
            }
            AddErrors(result);
        }

        // If we got this far, something failed, redisplay form
        return View(model);
    }

My ViewModel is:

 [Required]
    [Display(Name = "Membership Type")]
    public string SelectedMembershipType { get; set; }

    public List<MembershipTypeViewModel> MembershipType { get; set; }

Register.cshtml

 <div class="form-group">
                @Html.LabelFor(m => m.SelectedMembershipType, new { @class = "col-md-2 control-label" })
                <div class="col-md-10">
                    @Html.DropDownListFor(viewModel => viewModel.SelectedMembershipType, new SelectList(Model.MembershipType, "Id", "Name"), " Select Membership ", new { @class = "form-control" })
                </div>
            </div>