I think the issue is with the code in your second while loop. Notice that you are looking in the _reader for the 'Cname' field which isn't in this reader...
while (_reader1.Read())
{
Cname = _reader["Cname"].ToString();
treeItem.Items.Add(new TreeViewItem() { Header = Cname });
ClientTree.Items.Add(treeItem);
}
...I am pretty sure you want to call the second reader (_reader1) in your second while loop. Like so...
while (_reader1.Read())
{
Cname = _reader1["Cname"].ToString();
treeItem.Items.Add(new TreeViewItem() { Header = Cname });
ClientTree.Items.Add(treeItem);
}