System.nullreferenceexception：对象引用未设置为vb中对象的实例

Question

运行程序时出现此错误。 System.nullreferenceexception: object reference not set to an instance of an object。看来错误指向查询数据库的部分。这是代码

 cmdx.Connection = cnn
 cmdx.CommandText = "SELECT farmer_centre from farmer WHERE farmer_num ='" & fnumDField.Text.Trim & "'"

        myDatad = cmdx.ExecuteReader

        While myDatad.Read()
            Dim dbCentre As String = myDatad(0).ToString
        End While

我将dbCentre全局声明为     Dim dbCentre As String。 我该怎么办？我使用的是vb而数据库是访问