Mocking in C# Unit Tests error : ArgumentNullException: Value cannot be null. (Parameter 'source')

Question

I am new to unit testing and I've been facing this error that I can't even understand what it means.

So I have a solution called HotelApp, working perfectly with no issues. Now I am trying to unit test it. I am trying to test my Data access. I used Dapper for my data access and xUnit, Moq and Autofac for my testing.

**below is the data access class and interface. **

public class SqlDataAccess : ISqlDataAccess
    {
        private readonly IConfiguration _config;

        public SqlDataAccess(IConfiguration config)
        {
            _config = config;
        }

        public List<T> LoadData<T, U>(string sqlStatement,
                                      U parameter,
                                      string connectionStringName,
                                      bool isStoredProcedure = false)
        {
            string? connectionString = _config.GetConnectionString(connectionStringName);
            CommandType commandType = CommandType.Text;

            if (isStoredProcedure == true)
            {
                commandType = CommandType.StoredProcedure;
            }

            using (IDbConnection connection = new SqlConnection(connectionString))
            {
                List<T> rows = connection.Query<T>(sqlStatement, parameter, commandType: commandType).ToList();
                return rows;
            }

        }
}

public interface ISqlDataAccess
    {
        List<T> LoadData<T, U>(string sqlStatement, U parameter, string connectionStringName, bool isStoredProcedure = false);
    }

**below is the data access methode and interface. **

public class SqlData : IDatabaseData
    {
        private readonly ISqlDataAccess _db;
        private const string connectionStringName = "SqlDb";

        public SqlData(ISqlDataAccess db)
        {
            _db = db;
        }
public RoomTypesModel GetRoomTypesDetailById(int roomTypeId)
        {
            RoomTypesModel model= new RoomTypesModel();

             model = _db.LoadData<RoomTypesModel, dynamic>("[dbo].[spRoomTypeDetails_GetById]",
                                                          new { RoomTypeId = roomTypeId },
                                                          connectionStringName,
                                                          true).First();
            return model;
        }
}

**below is my unit test code. **

public class SqlDataTests : MockData
    {
        private static DateTime pStartDate = DateTime.Now;
        private DateTime pEndDate = pStartDate.AddDays(1);

        private const string connectionStringName = "SqlDb";
        
      [Fact]
        public void GetRoomTypesDetailById_ShouldReturnRoomDetails()
        {
            var roomTypeId = 2;
            var returnedRoom = GetAvailableRoomTypes()[1] ;
            var sqlStatment = "[dbo].[spRoomTypeDetails_GetById]";
            List<RoomTypesModel> roomList = new List<RoomTypesModel>();
            roomList.Add(returnedRoom);

            using (var mock = AutoMock.GetLoose())
            {
                mock.Mock<ISqlDataAccess>()
                        .Setup(x => x.LoadData<RoomTypesModel, dynamic>(sqlStatment, roomTypeId, connectionStringName, true))
                        .Returns(roomList);

                var _dbMock = mock.Create<SqlData>();

                // Arrange 
                var expected = returnedRoom;

                //Act 
                var actual = _dbMock.GetRoomTypesDetailById(roomTypeId);

                // Assurt
                Assert.NotNull(actual);
                Assert.Equal(actual, expected);
            }
        }
    }

**below is my MockData clase code. **

public class MockData
    {
       public List<RoomTypesModel> GetRoomTypesDetailById(int roomId) 
        {
            List<RoomTypesModel> RoomDetails = new()
            {
                new RoomTypesModel { Id = 1, Title = "King Size Bed", Description = "A room with a king size bed and a window view.", Price = 100 },
                new RoomTypesModel { Id = 2, Title = "Two Queen Size Ben", Description = "A room with two queen size bed", Price = 115 },
                new RoomTypesModel { Id = 3, Title = "Execlusive Suite", Description = "Two room, each with one king size bed and a window view", Price = 205 }
            };

            return RoomDetails.Where(room => room.Id == roomId).ToList();
        }
    }

This is a photo of my project to better understand it

Now I am expecting (actual) to retun an object with Id = 2 and the test to pass since I have no errors in the project and when I build. But when I run the test in debudding mode I keep getting the below error when It reaches the act part

var actual = _dbMock.GetRoomTypesDetailById(roomTypeId);

Error Detials:

System.Reflection.TargetInvocationException
  HResult=0x80131604
  Message=Exception has been thrown by the target of an invocation.
  Source=System.Private.CoreLib
  StackTrace:
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Span`1& arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) in /_/src/coreclr/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.cs:line 435
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters) in /_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBase.cs:line 53
   at Xunit.Sdk.TestInvoker`1.CallTestMethod(Object testClassInstance) in /_/src/xunit.execution/Sdk/Frameworks/Runners/TestInvoker.cs:line 150
   at Xunit.Sdk.TestInvoker`1.<>c__DisplayClass48_0.<<InvokeTestMethodAsync>b__1>d.MoveNext() in /_/src/xunit.execution/Sdk/Frameworks/Runners/TestInvoker.cs:line 257
   at Xunit.Sdk.ExecutionTimer.<AggregateAsync>d__4.MoveNext() in /_/src/xunit.execution/Sdk/Frameworks/ExecutionTimer.cs:line 48
   at Xunit.Sdk.ExceptionAggregator.<RunAsync>d__9.MoveNext() in /_/src/xunit.core/Sdk/ExceptionAggregator.cs:line 90

  This exception was originally thrown at this call stack:
    System.Linq.ThrowHelper.ThrowArgumentNullException(System.Linq.ExceptionArgument)
    System.Linq.Enumerable.TryGetFirst<TSource>(System.Collections.Generic.IEnumerable<TSource>, out bool)
    System.Linq.Enumerable.First<TSource>(System.Collections.Generic.IEnumerable<TSource>)
    HotalAppLibrary.Data.SqlData.GetRoomTypesDetailById(int) in SqlData.cs
    HotelAppLibrary.Tests.SqlDataTests.GetRoomTypesDetailById_ShouldReturnRoomDetails() in SqlDataTests.cs

Inner Exception 1:
ArgumentNullException: Value cannot be null. (Parameter 'source')