Error on InsertOneAsync: MongoDB.Driver.MongoWriteException

I am using C# and mongoDB Drivers, when I insert one document to the collection it is working good, but, when I try to insert a second I get this error:

MongoDB.Driver.MongoWriteException
HResult=0x80131500
Message=A write operation resulted in an error. WriteError: { Category : “DuplicateKey”, Code : 11000, Message : “E11000 duplicate key error collection: BB_MC_DEV.tbl_template_types index: id_1 dup key: { id: null }” }.
Source=MongoDB.Driver
StackTrace:
at MongoDB.Driver.MongoCollectionBase`1.d__76.MoveNext()
at BBMC.API.ManagePackageOrders.Repository.Services.TemplateTypeRepository.d__3.MoveNext() in C:\AllFiles\Repos2\BBMC\BackEnd\BBMC.API.ManagePackageOrders\Repository\Services\TemplateTypeRepository.cs:line 29

This exception was originally thrown at this call stack:
[External Code]

Inner Exception 1:
MongoBulkWriteException`1: A bulk write operation resulted in one or more errors. WriteErrors: [ { Category : “DuplicateKey”, Code : 11000, Message : “E11000 duplicate key error collection: BB_MC_DEV.tbl_template_types index: id_1 dup key: { id: null }” } ].

This is my DTO:

public class TblTemplateTypesDTO
{
[BsonId(IdGenerator = typeof(ObjectIdGenerator))]
[BsonRepresentation(BsonType.ObjectId)]
public ObjectId? id { get; set; }
public string? type { get; set; }
public string title { get; set; }
public int corefact_product_id { get; set; }
public string? corefact_product_code { get; set; }
public DateTime created { get; set; }
}

Here is the method of insertion:

public async Task<string> CreateTemplateType(TemplateTypes templateType)
{
    try
    {
        templateType.id = ObjectId.GenerateNewId();
        await _templateType.InsertOneAsync(templateType);
        return templateType.id.ToString();
    }
    catch (Exception ex)
    {
        _logger.LogError("TemplateTypeRepository CreateTemplateType() Log => {@result}", ex.Message);
        throw new ExceptionCreate(ex, "TemplateTypes");
    }
}