Treeview for database objects C#

I tried the first link and i am getting the following message "not all code paths return a value", any ideas to why i am seeing this message ?

public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{
CreateDataTable();
}

public DataTable CreateDataTable()
{
DataTable dataTable = new DataTable();
dataTable.Columns.Add("Country");
// The value in this column will display on the TreeNode
dataTable.Columns.Add("City");
// The value in this column will identify its parentId

// Fill the DataTable
dataTable.Rows.Add("India", "New Delhi");
dataTable.Rows.Add("India", "Mumbai");
dataTable.Rows.Add("India", "Kolkata");
dataTable.Rows.Add("India", "Noida");
dataTable.Rows.Add("USA", "New York");
dataTable.Rows.Add("USA", "Washington");
dataTable.Rows.Add("USA", "");
dataTable.Rows.Add("USA", "India");
return dataTable;
}

public void BuildTree(DataTable dt, TreeView trv, Boolean expandAll)
{
// Clear the TreeView if there are another datas in this TreeView
trv.Nodes.Clear();
TreeNode node = default(TreeNode);
TreeNode subNode = default(TreeNode);
foreach (DataRow row in dt.Rows)
{
//search in the treeview if any country is already present
node = Searchnode(row[0].ToString(), trv);
if (node != null)
{
//Country is already present
subNode = new TreeNode(row[1].ToString());
//Add cities to country
node.Nodes.Add(subNode);
}
else
{
node = new TreeNode(row[0].ToString());
subNode = new TreeNode(row[1].ToString());
//Add cities to country
node.Nodes.Add(subNode);
trv.Nodes.Add(node);
}
}
if (expandAll)
{
// Expand the TreeView
trv.ExpandAll();
}
}
public TreeNode Searchnode(string nodetext, TreeView trv)
{
foreach (TreeNode node in trv.Nodes)
{
if (node.Text == nodetext)
{
return node;
}
}

return null;
}
}


I need to call BuildTree in order to have a value isn't that correct ? since the value returned is null in this case