Csharp 如何解决提交按钮后 Select 选项错误空参照异常

我正在处理一个涉及ASP.NET Core 7.0中SELECT选项的使用的项目，我遇到了一些困难.具体地说，我在下拉列表中遇到了与空引用异常相关的错误.然而，我观察到，提交数据后，它被安装到SQL数据库中，提交后返回时，显示空引用异常错误.这些错误使开发过程变得非常具有挑战性，我希望尽快找到解决方案.如有任何意见或建议，将不胜感激.

Error

Action个

public IActionResult Apply()
{

    ViewBag.Courselist = new SelectList(GetAllAccountIDList(), "Id", 
    "CourseName");
    return View();
}
[HttpPost]
public IActionResult Apply(PurchasedFormViewModel purchasedFormVM)
{
    if (ModelState.IsValid)
    {
    try
    {
        var transaction = new StudentBiodataViewModels
        {
            CourseId = purchasedFormVM.CourseId,
            SurName = purchasedFormVM.SurName,
            OtherName = purchasedFormVM.OtherName,
            PhoneNumber = purchasedFormVM.PhoneNumber,
            Email = purchasedFormVM.Email,
        };
        _context.StudentBiodata.Add(transaction);
        _context.SaveChangesAsync();
    }
    catch (Exception ex)
    {

        ViewBag.Message = ex.Message + " Error occur while inserting records.";
    }
    }
    return View();
}

Models个

 public class ApplyFormViewModel
 { 
     public int Id { get; set; }
     [Display(Name = "Course Apply")]
     [Required(ErrorMessage = "Course Apply is Required")]
     public int CourseId { get; set; }
     [Display(Name = "SurName")]
     [Required(ErrorMessage = "SurName is Required")]
     public string SurName { get; set; }
     [Display(Name = "Other Name")]
     [Required(ErrorMessage = "Other Name is Required")]
     public string OtherName { get; set; } 
     [Display(Name = "Phone Number")]
     [Required(ErrorMessage = "Phone Number is Required")]
     ErrorMessage = "Entered phone format is not valid.")]
     public string PhoneNumber { get; set; } 
     [Required(ErrorMessage = "Email is Required")]
     public string Email { get; set; }
     
 }

View个

<select id="fundcode" asp-for="CourseId" class="form-control">
    <option value="">--Please select--</option>
    @foreach (var item in ViewBag.Courselist )
    {
        <option value="@item.Value">@item.Text</option>
    }
</select>