ASP.NET Core Web API - Object reference not set to an instance of an object

Question

I am implementing Repository and Unit of Work in ASP.NET Core-6 Web API. i HAVE THIS CODE:

public interface IGenericRepository<T> where T : class
{
    T GetById(string id);

    IEnumerable<T> GetAll();

    IEnumerable<T> Find(Expression<Func<T, bool>> expression);

    void Add(T entity);
    Task InsertAsync(T entity);

    void AddRange(IEnumerable<T> entities);

    void Delete(T entity);

    void DeleteRange(IEnumerable<T> entities);

    void Update(T entity);
}

AND THE IMPLEMENTATION:

public class GenericRepository<T> : IGenericRepository<T> where T : class
{
    private readonly ApplicationDbContext _dbContext;
    private readonly DbSet<T> _dbSet;

    public GenericRepository(ApplicationDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public void Add(T entity)
    {
        _dbContext.Set<T>().Add(entity);
    }
    public async Task InsertAsync(T entity)
    {
        await _dbSet.AddAsync(entity);
    }

    public void AddRange(IEnumerable<T> entities)
    {
        _dbContext.Set<T>().AddRange(entities);
    }

    public IEnumerable<T> Find(Expression<Func<T, bool>> expression)
    {
        return _dbContext.Set<T>().Where(expression);
    }

    public IEnumerable<T> GetAll()
    {
        return _dbContext.Set<T>().ToList();
    }

    public T GetById(string id)
    {
        return _dbContext.Set<T>().Find(id);
    }

    public void Delete(T entity)
    {
        _dbContext.Set<T>().Remove(entity);
    }

    public void DeleteRange(IEnumerable<T> entities)
    {
        _dbContext.Set<T>().RemoveRange(entities);
    }

    public void Update(T entity)
    {
        _dbContext.Set<T>().Update(entity);
    }
}

Also I have the Repository:

Interface:

public interface IBabRepository : IGenericRepository<BabTransactionDetail>
{
}

Implementation:

public class BabRepository : GenericRepository<BabTransactionDetail>, IBabRepository
{
    private readonly ApplicationDbContext _dbContext;
    private readonly DbSet<BabTransactionDetail> _babTransactions;

    public BabRepository(ApplicationDbContext dbContext) : base(dbContext)
    {
        _dbContext = dbContext;
        _babTransactions = _dbContext.Set<BabTransactionDetail>();
    }
}

Unit of Work:

public interface IUnitOfWork : IDisposable
{
    IBabRepository BabTransactionDetails { get; }
    Task Save();
}

public class UnitOfWork : IUnitOfWork
{
    private IBabRepository _babTransactions;
    private readonly ApplicationDbContext _dbContext;

    public UnitOfWork(ApplicationDbContext dbContext)
    {
        _dbContext = dbContext;
    }
    public IBankUserRepository BankUsers => _bankUsers ??= new BankUserRepository(_dbContext);
    public IBabRepository BabTransactionDetails => _babTransactions ??= new BabRepository(_dbContext);

    public async Task Save()
    {
        await _dbContext.SaveChangesAsync();
    }

    public void Dispose()
    {
        _dbContext.Dispose();
        GC.SuppressFinalize(this);
    }
}

Finally, the service:

public interface IBabService
{
    Task<BaseResponse> MyAccount(InputRequest request);
}

public async Task<BaseResponse> MyAccount(InputRequest payload)
{
    var response = new BaseResponse();
    try
    {
        var babData = new BabTransactionDetail
        {
            UserName = payload.user_name,
            UserEmail = payload.user_email,
            UserPhone = payload.user_phone,
            Amount = payload.amount,
        };
     //   await _unitOfWork.BabTransactionDetails.InsertAsync(babData);
      //  await _unitOfWork.Save();
            _dbContext.BabTransactionDetails.Add(babData);
            await _dbContext.SaveChangesAsync();
 }

When I submitted, I got this error:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Application
  StackTrace:
   at ApplicationRepositories.Concrete.GenericRepository`1.<InsertAsync>d__4.MoveNext()

and points at InsertAsync(T entity)

I checked the models and entries, there is data.

Where am I getting it wrong, and how do I resolve this?

Thanks