how to know the reason of error System.NullReferenceException: 'Object reference not set to an instance of an object.'

Question

I have the following subroutine to get data from table:

'GetData
    Public Sub GetData(ByVal SqlStr As String, ByVal xDt As DataTable, ByVal xPar() As MySqlParameter)
        Using xCmd As New MySqlCommand() With {
                    .CommandType = CommandType.Text,
                    .CommandText = SqlStr,
                    .Connection = _ServerConnStr
                    }

            If xPar IsNot Nothing Then
                For i As Integer = 0 To xPar.Length - 1
                    xCmd.Parameters.Add(xPar(i))
                Next
            End If

            Using xDa As New MySqlDataAdapter(xCmd)
                xDa.Fill(xDt)
            End Using
            xDt.Dispose()
        End Using
    End Sub

and I use that sub to get data from tblfatora1:

Public Sub MySql_GetFhMain()
        xDtMain = New DataTable()

        Dim xPar(2) As MySqlParameter
        xPar(0) = New MySqlParameter("@Date1", MySqlDbType.Date) With {
                                    .Value = Date1}
        xPar(1) = New MySqlParameter("@Date2", MySqlDbType.Date) With {
                                    .Value = Date2}
        xPar(2) = New MySqlParameter("@FhReso", DbType.Int32) With {
                                    .Value = FhReso}

            xSqlStr = "SELECT 
                FhID, FhRef, FhCode, FhDate, FhQuan, FhPurPrice,  
                FhPurTotal, FhCarNo, FhResoDetails, FhPalletQuan, 
                FhPalletPrice, accreso.AccName AS ResoName, 
                accdriv.AccName AS DriverName, CONCAT(CategoryName,'-',ProductName) AS ProductName,

                (SELECT GROUP_CONCAT(AccName) FROM tblaccounts 
                    INNER JOIN tblfatora2 ON 
                        tblfatora2.FbCus = tblaccounts.AccID 
                                where FIND_IN_SET(AccID, FbCus) AND tblfatora2.FhRef = tblfatora1.FhRef)AS TheCustomers,

                (SELECT GROUP_CONCAT(PlaceName) FROM tblplaces 
                    INNER JOIN tblfatora2 ON 
                        tblfatora2.FbCusPlace = tblplaces.PlaceID 
                                where FIND_IN_SET(PlaceID, FbCusPlace) AND tblfatora2.FhRef = tblfatora1.FhRef)AS ThePlaces , 

                (FhQuan -  (SELECT IFNULL(SUM(FbQuan), 0) FROM tblfatora2 WHERE FhRef = tblfatora1.FhRef)) AS TheRemain, 
ProductPallet, PlaceName, FhDriver, FhProduct, FhReso, FhResoPlace     
                                       
                                            FROM tblfatora1
                                        INNER JOIN tblproducts ON tblproducts.ProductID = tblfatora1.FhProduct
                                        INNER JOIN tblaccounts accreso ON accreso.AccID = tblfatora1.FhReso
                                        INNER JOIN tblaccounts accdriv ON accdriv.AccID = tblfatora1.FhDriver  
                                        INNER JOIN tblcategories cat ON cat.CategoryID = tblproducts.ProductCategory 
                                        INNER JOIN tblcurrencies curr ON accdriv.AccCurrID = curr.CurrencyID
                                        INNER JOIN tblplaces plc ON plc.PlaceID = tblfatora1.FhResoPlace 
                                        WHERE (FhDate Between @Date1 And @Date2)"
       

        xClsMySql.GetData(xSqlStr, xDtMain, xPar)

    End Sub

but sometimes when I run it I get the following error:

although when I check the rows count I see that there are many rows so the DtMain has rows:

as I said that error sometimes happens, not always, I tried to see the reason of the error but I couldn't