The error message "Top-level control cannot be added to a control" is self-explanatory.

It's easy to fix. Before you add a top-level control to a control, you need to set top level to false:



Then you need to show the control, using Show(), after it's added to a control.

You can test whether the control is a top level control:


When the control is not a top level control, it looses all the form level events such as Close().

Since the control is no longer the top level control, it doesn't look like a separate form. To compensate for this, you can add all your controls in a Panel.