关于c＃：异常：尝试在dataSet中的表之间创建关系时出现System.NullReferenceException

Exceptions: System.NullReferenceException while trying to create relation between tables in dataSet

使用C＃、. net Framework 4.5，VS 2012

尝试在数据集中的表之间创建简单关系，但是遇到System.NullReferenceException，正如我在MSDN上看到的那样，它的意思是occurs when you try to reference an object in your code that does not exist。但是，认为我创建了所有必需的对象。

我的代码如下：

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//create place for storing all tables from data base
private DataSet myDS = new DataSet("AutoLot");

//command builders for easy way access to tables
private SqlCommandBuilder sqlCInventory;
private SqlCommandBuilder sqlCOrders;
private SqlCommandBuilder sqlCCustomers;

//adapters for each table
private SqlDataAdapter sqlAInventory;
private SqlDataAdapter sqlAOrders;
private SqlDataAdapter sqlACustomers;

//connection string
private string cnStr = string.Empty;

public MainForm()
{
InitializeComponent();
//get connection string from .config file
cnStr =
ConfigurationManager.ConnectionStrings["AutoLotSqlProvider"].ConnectionString;
//create adapters
sqlACustomers = new SqlDataAdapter("Select * From Customers", cnStr);
sqlAInventory = new SqlDataAdapter("Select * From Inventory", cnStr);
sqlAOrders = new SqlDataAdapter("Select * From Orders", cnStr);

//automatic generate commands
sqlCCustomers = new SqlCommandBuilder(sqlACustomers);
sqlCInventory = new SqlCommandBuilder(sqlAInventory);
sqlCOrders = new SqlCommandBuilder(sqlAOrders);

//add table to data Set
sqlAInventory.Fill(myDS);
sqlAOrders.Fill(myDS);
sqlACustomers.Fill(myDS);

//create relationship between tables
BuildTableRelationShip();

//create DataSourse for datGrids on UI
dataGridViewCustomer.DataSource = myDS.Tables["Inventory"];
dataGridViewOrders.DataSource = myDS.Tables["Orders"];
dataGridViewCustomer.DataSource = myDS.Tables["Customers"];
}

这是我的例外

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private void BuildTableRelationShip()
{
//create object of relationShips
DataRelation dr = new DataRelation("CustomersOrders", //name of relation
myDS.Tables["Customers"].Columns["CustID"], //main columns
myDS.Tables["Orders"].Columns["OrderID"]); //related columns
myDS.Relations.Add(dr);

//second relation
dr = new DataRelation("InventoryOrder",
myDS.Tables["Inventory"].Columns["CarID"],
myDS.Tables["Orders"].Columns["OrderID"]);
//add relations to dataset
myDS.Relations.Add(dr);
}

为什么我收到此Null参考异常？我想念什么？

编辑

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