asp.net-core - 具有外键列（复杂对象）的 blazor 服务器（blazorise 数据网格）

Question

我将 blazor (blazorise datagrid) 与 asp.net 核心和实体框架一起使用。我在尝试对具有外键列（复杂对象）的数据执行 crud 操作时遇到问题。这是我的代码

模型类

public partial class ApproverSequence
    {
        public int Id { get; set; }
        public int? TransactionTypeId { get; set; }
        public bool? IsStart { get; set; }
        public bool? IsArchived { get; set; }
        public virtual TransactionType TransactionType { get; set; }
    }
   public partial class TransactionType
    {
        public TransactionType()
        {
            ApproverSequences = new HashSet<ApproverSequence>();
        }
        public int Id { get; set; }
        public string Description { get; set; }
        public bool? IsArchived { get; set; }
        public virtual ICollection<ApproverSequence> ApproverSequences { get; set; }
    }

这是我的数据网格列

<DataGridSelectColumn TItem="ApproverSequence" Field="@nameof(ApproverSequence.TransactionType)" Caption="Description" Editable="true">
            <DisplayTemplate>
                @{
                    var transactionTypeDesc = (context as ApproverSequence).TransactionType?.Description;
                    @transactionTypeDesc
                }
            </DisplayTemplate>
            <EditTemplate>
                <Select TValue="int"
                    SelectedValue="@((int)((TransactionType)(((CellEditContext)context).CellValue)).Id)"
                    SelectedValueChanged="@( v => ((CellEditContext)context).CellValue = transactionTypes.First(x=> x.Id == v))">
                    @foreach (var item in transactionTypes)
                        {
                            <SelectItem TValue="int" Value="@(item.Id)">@item.Description</SelectItem>
                        }
                </Select>
            </EditTemplate>
</DataGridSelectColumn>
@code{
 private List<ApproverSequence> approverSequences;
 private List<TransactionType> transactionTypes;

    protected override async Task OnInitializedAsync()
    {
        approverSequences = await ApproverSequenceService.GetAll();
        transactionTypes = await TransactionTypeService.GetAll();
    }
}

服务

public async Task<List<ApproverSequence>> GetAll()
        {
            try
            {
                return await _context.ApproverSequences.Include(x=>x.TransactionType).Where(x => x.IsArchived == false).ToListAsync();
            }
            catch
            {
                throw;
            }
        }
 public async Task<List<TransactionType>> GetAll()
        {
            try
            {
                return await _context.TransactionTypes.Where(x => x.IsArchived == false).ToListAsync();
            }
            catch
            {
                throw;
            }
        }

运行此项目时，它会抛出 System.NullReferenceException: 'Object reference not set to an instance of an object。例外。有什么我错过的吗？提前致谢