C# – LINQ – Association mapping – Null reference on InsertOnSubmit

I've a table Role associated in 1-many relation with table User in my database. I've created LINQ mapping classes manually:

[Table(Name="Role")]
public class Role
{
    private EntitySet<User> _Users;

    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert)]
    public int RoleID { get; set; }
    [Column] public string Name { get; set; }
    [Association(Name = "FK_User_Role", Storage = "_Users", ThisKey = "RoleID", OtherKey = "RoleID")]
    public EntitySet<User> Users
    {
        get{ return this._Users; }
        set{ this._Users.Assign(value);}
    }
}

The problem is that EntitySet Users can't be nullable so if later than I wish to create new role:

public override void CreateRole(string roleName)
{
    try
    {
        Role new_role = new Role();
        new_role.Name = roleName;
        _RolesRepository.SaveRole(new_role);
    }
    catch
    {
        throw;
    }
}

I'm getting error message listed below:

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not
set to an instance of an object.

Source Error:

Line 36: else Line 37: { Line 38:
rolesTable.InsertOnSubmit(role); ins = true; Line 39: }
Line 40:

Source File:
C:\inetpub\sklepomat\DomainModel\Concrete\SqlRolesRepository.cs
Line: 38

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an
object.]
System.Data.Linq.Mapping.EntitySetDefSourceAccessor2.GetValue(T
instance) +15
System.Data.Linq.Mapping.MetaAccessor2.GetBoxedValue(Object instance)
+44 System.Data.Linq.StandardTrackedObject.HasDeferredLoader(MetaDataMember
deferredMember) +90
System.Data.Linq.StandardTrackedObject.get_HasDeferredLoaders() +102
System.Data.Linq.StandardChangeTracker.Track(MetaType mt, Object obj,
Dictionary2 visited, Boolean recurse, Int32 level) +187
System.Data.Linq.StandardChangeTracker.Track(Object obj, Boolean
recurse) +80 System.Data.Linq.StandardChangeTracker.Track(Object
obj) +9 System.Data.Linq.Table1.InsertOnSubmit(TEntity entity)
+172 DomainModel.Concrete.SqlRolesRepository.SaveRole(Role role) in C:\inetpub\sklepomat\DomainModel\Concrete\SqlRolesRepository.cs:38
DomainModel.Concrete.SklepomatRoleProvider.CreateRole(String roleName)
in
C:\inetpub\sklepomat\DomainModel\Concrete\SklepomatRoleProvider.cs:71
System.Web.Security.Roles.CreateRole(String roleName) +73
WebUI.Controllers.TempController.ble() in
C:\inetpub\sklepomat\WebUI\Controllers\TempController.cs:29
lambda_method(ExecutionScope , ControllerBase , Object[] ) +74
System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase
controller, Object[] parameters) +17
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext
controllerContext, IDictionary2 parameters) +178
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext
controllerContext, ActionDescriptor actionDescriptor, IDictionary2
parameters) +24
System.Web.Mvc.<>c__DisplayClassa.b__7()
+52 System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter
filter, ActionExecutingContext preContext, Func1 continuation) +254
System.Web.Mvc.<>c__DisplayClassc.<InvokeActionMethodWithFilters>b__9()
+19 System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext
controllerContext, IList1 filters, ActionDescriptor actionDescriptor,
IDictionary`2 parameters) +192
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext
controllerContext, String actionName) +399
System.Web.Mvc.Controller.ExecuteCore() +126
System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
+27 System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext
requestContext) +7
System.Web.Mvc.MvcHandler.ProcessRequest(HttpContextBase httpContext)
+151 System.Web.Mvc.MvcHandler.ProcessRequest(HttpContext httpContext) +57
System.Web.Mvc.MvcHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext
httpContext) +7
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
+181 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

I assume that's because EntitySet<User> _Users equals to null…
Am I right !?
What should be assigned to _Users if it can't by nullable !?

UPDATE:

Exception is raised

        try { rolesTable.InsertOnSubmit(role);}
        catch (Exception ex)
        {
            throw ex;

        }

Where role object's fields are set to:

_Users = null,
Name = "New name",
RoleID = 0,
Users = null