Conversion from type 'DBNull' to type 'String'

Private Sub BackupMySql()

        Dim localDir As String = "C:/"
        Dim strDate As String = Date.Now.ToShortDateString   'Prepend file with date for dated backups
        'Dim fileName As String = strDate.Replace("/", "-") & "_" & ftphost & ".sql"
        Dim fileName As String = strDate.Replace("/", "-") & "_" & "test" & ".sql"
        Dim saveFile As String = localDir & fileName
        Dim DBServer As String = "localhost"
        Dim DBServerPort As String = "3306"
        Dim Database As String = "dtr"
        Dim DBUser As String = "root"
        Dim DBPass As String = "pinoy"
        Dim MyConString As String = "SERVER=" & DBServer & ";DATABASE=" & Database & ";UID=" & DBUser & ";PASSWORD=" & DBPass & ";PORT=" & DBServerPort & ";charset=utf8"
        Dim connection As New MySqlConnection(MyConString)
        Dim tablesCommand As MySqlCommand = connection.CreateCommand()
        Dim rowsCommand As MySqlCommand = connection.CreateCommand()
        Dim tablesReader As MySqlDataReader
        Dim rowsReader As MySqlDataReader

        tablesCommand.CommandText = "SHOW TABLES FROM " & Database
        connection.Open()
        tablesReader = tablesCommand.ExecuteReader()

        Dim query As String = ""
        Dim tablesArray As New ArrayList()

        While tablesReader.Read()
            tablesArray.Add(tablesReader.GetValue(0).ToString())
        End While

        tablesReader.Close()
        For i = 0 To tablesArray.Count - 1
            query += "DROP TABLE IF EXISTS `" & tablesArray(i) & "`;" & vbCrLf '& Database & "."
            'query += "DROP TABLE IF EXISTS `" & tablesArray(i) & "`;" & vbCrLf '& Database & "."
            query += vbCrLf & "CREATE TABLE `" & tablesArray(i) & "` (" & vbCrLf
            rowsCommand.CommandText = "DESCRIBE " & tablesArray(i)
            rowsReader = rowsCommand.ExecuteReader()

            Dim temp As String = ""
            While rowsReader.Read()
                query += "`" & rowsReader.GetString("Field") & "` " & rowsReader.GetString("Type")
                If Not rowsReader.GetString("Null") = "YES" Then
                    query += " NOT NULL"
                End If
                If IsDBNull(rowsReader.Item("Default")) = False Then
                    query += " DEFAULT '" & rowsReader.GetString("Default") & "'"
                End If
                If Not rowsReader.GetString("Extra") = Nothing Then
                    query += " " & rowsReader.GetString("Extra").ToUpper()
                End If
                If rowsReader.GetString("Key") = "PRI" Then
                    temp = "primary key(" & rowsReader.GetString("Field") & ")"
                End If
                query += "," & vbCrLf

            End While
            query += temp & vbCrLf & ");" & vbCrLf & vbCrLf
            rowsReader.Close()
            rowsCommand.CommandText = "SELECT * FROM " & tablesArray(i)
            rowsReader = rowsCommand.ExecuteReader()

            While rowsReader.Read()
                query += "INSERT INTO `" & tablesArray(i) & "` ("
                Dim count As Integer = rowsReader.FieldCount - 1
                Dim keys(count) As String
                Dim values(count) As String
                For n = 0 To count
                    keys(n) = rowsReader.GetName(n)
                    values(n) = rowsReader.Item(n)
                Next
                query += Join(keys, ", ") & ")" & vbCrLf & "VALUES ('" & Join(values, "', '") & "');" & vbCrLf
            End While
            rowsReader.Close()
            query += vbCrLf & vbCrLf

        Next
        connection.Close()
        connection.Dispose()

        If File.Exists(saveFile) Then
            File.Delete(saveFile)
        End If
        Dim objWriter As New System.IO.StreamWriter(saveFile)

        objWriter.Write(query)
        objWriter.Close()

    End Sub