C# 添加到实体中的集合

我有一个班级学校，可以举办很多活动

public class School: Entity<Guid>
{
    public string Name { get; set; }
    public ICollection<Event> Events { get; set; } = new List<Event>();
    public void AddEvent(Event newEvent) 
    {
        Events.Add(newEvent);
    }
}

公立学校：实体
{
公共字符串名称{get；set；}
公共ICollection事件{get；set；}=new List（）；
公共无效添加事件（事件newEvent）
{
Events.Add（newEvent）；
}
}

这是事件类

public class Event : Entity<Guid>
{
    public string Title { get; set; }
    public string Description { get; set; }
    public DateTime EventStart { get; set; }
}

公共类事件：实体
{
公共字符串标题{get；set；}
公共字符串说明{get；set；}
公共日期时间事件开始{get；set；}
}

这是一个实体类-不相关，但如果有人想看到

public abstract class Entity<TId> 
{
    public TId Id { get; set; }
}

公共抽象类实体
{
公共TId Id{get；set；}
}

当我想为学校添加一个新事件时，我用事件类的属性调用一个POST端点

[HttpPost]
public async Task<IActionResult> CreateEvent(Guid schoolId, Event newEvent)
{
     var school = await _context.School.FirstOrDefaultAsync(x => x.Id == schoolId);
     
     school.AddEvent(newEvent);
     
     await _context.SaveChangesAsync();
     
     return Ok();
}

[HttpPost]
公共异步任务CreateEvent（Guid schoolId，Event newEvent）
{
var school=wait_context.school.FirstOrDefaultAsync（x=>x.Id==schoolId）；
学校。附加事件（新事件）；
wait_context.SaveChangesAsync（）；
返回Ok（）；
}

但是，由于某些原因，这会导致此错误

完整堆栈跟踪：

System.ArgumentNullException: Value cannot be null. (Parameter 'key')
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityReferenceMap.TryGet(Object entity, IEntityType entityType, InternalEntityEntry& entry, Boolean throwOnNonUniqueness)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.TryGetEntry(Object entity, IEntityType entityType, Boolean throwOnTypeMismatch)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup(InternalEntityEntry entry, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.FireStateChanged(EntityState oldState)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState oldState, EntityState newState, Boolean acceptChanges, Boolean modifyProperties)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState entityState, Boolean acceptChanges, Boolean modifyProperties, Nullable`1 forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction(EntityEntryGraphNode`1 node)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TState](EntityEntryGraphNode`1 node, Func`2 handleNode)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TState](EntityEntryGraphNode`1 node, Func`2 handleNode)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TState](EntityEntryGraphNode`1 node, Func`2 handleNode)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph(InternalEntityEntry rootEntry, EntityState targetState, EntityState storeGeneratedWithKeySetTargetState, Boolean forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.NavigationCollectionChanged(InternalEntityEntry entry, INavigationBase navigationBase, IEnumerable`1 added, IEnumerable`1 removed)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.NavigationCollectionChanged(InternalEntityEntry entry, INavigationBase navigationBase, IEnumerable`1 added, IEnumerable`1 removed)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectNavigationChange(InternalEntityEntry entry, INavigationBase navigationBase)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.LocalDetectChanges(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectChanges(IStateManager stateManager)
   at Microsoft.EntityFrameworkCore.ChangeTracking.ChangeTracker.DetectChanges()
   at Microsoft.EntityFrameworkCore.DbContext.TryDetectChanges()
   at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
   at MakersOfDenmark.WebAPI.Controllers.EventController.CreateEvent(Guid schoolId, Event newEvent) in C:\Users\mathi\Source\Repos\makersofdenmark\src\MakersOfDenmark.WebAPI\Controllers\EventController.cs:line 50
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfIActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Logged|12_1(ControllerActionInvoker invoker)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Serilog.AspNetCore.RequestLoggingMiddleware.Invoke(HttpContext httpContext)
   at Elmah.Io.AspNetCore.Serilog.ElmahIoSerilogMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

System.ArgumentNullException:值不能为null。（参数“key”）
at System.Collections.Generic.Dictionary`2.FindEntry（TKey）
在System.Collections.Generic.Dictionary`2.TryGetValue（TKey、TValue和value）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityReferenceMap.TryGet（对象实体、entityType entityType、InternalEntityEntry和entry、布尔Throwon非唯一性）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.TryGetEntry（对象实体，IEntityType entityType，布尔throwOnTypeMismatch）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup（InternalEntityEntry条目，布尔fromQuery）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.StateChanged（InternalEntityEntry条目、EntityState oldState、布尔fromQuery）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.StateChanged（InternalEntityEntry条目、EntityState oldState、布尔fromQuery）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.FireStateChanged（EntityState oldState）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState（EntityState oldState、EntityState newState、Boolean acceptChanges、Boolean modifyProperties）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState（EntityState EntityState、Boolean acceptChanges、Boolean modifyProperties、Nullable`1 forceStateWhenUnknownKey）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction（EntityEntryGraphNode`1节点）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TSState]（EntityEntryGraphNode`1节点，Func`2 handleNode）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TSState]（EntityEntryGraphNode`1节点，Func`2 handleNode）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TSState]（EntityEntryGraphNode`1节点，Func`2 handleNode）
在Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph（InternalEntityEntry rootEntry、EntityState targetState、EntityState StoreWithKeySetTargetState生成的EntityState、Boolean ForceState WhenUnknownkey）
在Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.NavigationCollectionChanged（InternalEntityEntry条目，INavigationBase navigationBase，添加了IEnumerable`1，删除了IEnumerable`1）
在Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.NavigationCollectionChanged（InternalEntityEntry条目，INavigationBase导航库，添加了IEnumerable`1，删除了IEnumerable`1）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectNavigationChange（InternalEntityEntry条目，INavigationBase navigationBase）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.LocalDetectChanges（InternalEntityEntry条目）
位于Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectChanges（IStateManager stateManager）
在Microsoft.EntityFrameworkCore.ChangeTracking.ChangeTracker.DetectChanges（）中
在Microsoft.EntityFrameworkCore.DbContext.TryDetectChanges（）中
在Microsoft.EntityFrameworkCore.DbContext.SaveChangesSync（布尔值AcceptillChangesOnSuccess，CancellationToken CancellationToken）
在C:\Users\mathi\Source\Repos\MakersOfDenmark\src\MakersOfDenmark.WebAPI\Controllers\eventcontrollers\EventController.CreateEvent（Guid学校ID，Event newEvent）中的MakersOfDenmark.WebAPI
位于Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfActionResultExecutor.Execute（IActionResultTypeMapper映射器、ObjectMethodExecutor执行器、对象控制器、对象[]参数）
在Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g|u记录| 12_1（ControllerActionInvoker invoker invoker）
在Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g_u等待| 10_0（ControllerActionInvoker invoker，Task lastTask，State next，Scope Scope，Object State，Boolean isCompleted）
位于Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow（ActionExecutedContextSealed上下文）
位于Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next（状态和下一步、范围和范围、对象和状态、布尔值和isCompleted）
在Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g_u等待| 13_0（ControllerActionInvoker invoker，Task lastTask，State next，Scope Scope，Object State，Boolean isCompleted）
在Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g_|19_0（ResourceInvoker invoker、Task lastTask、State next、Scope Scope、Object State、Boolean isCompleted）
public class School: Entity<Guid>
{
    public string Name { get; set; }
    public ICollection<Event> Events { get; set; } = new List<Event>();

    public void AddEvent(Event newEvent) 
    {
        Events.Add(newEvent);
    }
}

public class Event : Entity<Guid>
{
    public string Title { get; set; }
    public string Description { get; set; }
    public DateTime EventStart { get; set; }
    public School School { get;set; }
    public Guid SchoolId { get;set; }
}


//on your DB context
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Event>().HasOne(x => x.School).WithMany(x => x.Events);
}