Update demo

Src/Asp.Net/SqlServerTest/UnitTest/UCustom012.cs

@@ -81,8 +81,9 @@ namespace OrmTest
             db.ThenMapper(list.Select(it => it.SchoolA), sch =>
             {
                 //参数1: room表关联字段  参数2: school表关联字段，  参数3: school当前记录
+                if(sch!=null)
                   sch.RoomList = db.Queryable<RoomA>().SetContext(room => room.SchoolId, () => sch.SchoolId, sch).ToList();
-
+                if (sch != null)
                     sch.TeacherList = db.Queryable<TeacherA>().SetContext(teachera => teachera.SchoolId, () => sch.SchoolId, sch).ToList();
             });
      

Src/Asp.Net/SqlSugar/Abstract/QueryableProvider/QueryableProvider.cs

@@ -1332,6 +1332,10 @@ namespace SqlSugar
         }
         public List<T> SetContext<ParameterT>(Expression<Func<T, object>> thisFiled, Expression<Func<object>> mappingFiled, ParameterT parameter) 
         {
+            if (parameter == null) 
+            {
+                return new List<T>();
+            }
             List<T> result = new List<T>();
             var entity = this.Context.EntityMaintenance.GetEntityInfo<ParameterT>();
             var queryableContext = this.Context.TempItems["Queryable_To_Context"] as MapperContext<ParameterT>;