MDI application and docking forms

I tried to convert the delphi 5 docking demo ($delphi5dir\demos\docking)
into an MDI application. This
resulted in strange unwanted behaviour. (control contents disappearance)

I proceeded as follows:

* Set the mainform at formstyle fsMDIForm
* Compile and run the application
* Dock one of the windows
* Undock the same window
* Dock the same window again

After doing this, the contents of the active control on
the form being docked and undocked disappeared.

Now why is this?

Please help me....