Hi Clark,
I was able to see the problem and I have included the code to dispose the docking controls explicitly in the menu form closing event. And also I have included the DisposeControls public method in the MyTabPage user control which is used to dispose all the docking controls in the current invoking tab page control.
Please take a look at the attached sample which illustrates the above functionality and let me know if this helps you.
Thanks for your patience.
Regards,
Vijay
Modified Sample