WinForms panel crash with V8, works fine with V7

My plugin has a form made with WinForms. It works well with V7. After converting solution to .NET 7 for V8 and adjusted to build, Rhino crashes as I try to show my panel.
I use Rhino.UI.Panels.RegisterPanel that gives no error.
Looking at crash report I get error: “Hosted HWND must be a child window of the specified parent”.
Do I need to set MdiParent into my form constructor? How to set it?

SOLVED. Need to use UserControl instead of Form.