// this happens inside class MainForm : Form
x = new X(_node, _nodeList, this, tsLblMessage);

// constructor, X
public X(TreeNode node, List<TreeNode> nodeList, Form parentForm, ToolStripStatusLabel tssLabel)
{
    _node = node;
    _nodeList = nodeList;
    _parentForm = parentForm;
    _tssLabel = tssLabel;

    _y = new Y(_parentForm, _dataAnalysis, _node)
    {
        Dock = DockStyle.Fill
    };

    // This doesn't work
    // _parentForm.Controls.Add(_y);

    // This does work, but I don't want MDI
    _form = new Form { MdiParent = _parentForm };
    _form.Controls.Add(_y);
    _form.Show();
}

// constructor, Y : UserControl
public Y(Form parentForm, DataAnalysis dataAnalysis, TreeNode node)
{
    InitializeComponent();

    _parentForm = parentForm;
    _dataAnalysis = dataAnalysis;
    _node = node;

    treeView.Nodes.Add(_node);
}