Need help with a noob question

First of all, thank you for making this great GUI system!

I managed to implement MyGui in my ogre3D project (since my mouse cursor changes to the MyGui one)
But when I try to make a widget I get an assert error when running it.

After some hours searching for the problem I gave up :p
I hope someone can point me in the right direction.

Here are some screenshots.
This one is from teh assert error message window:


This one is my callstack:


I hope someone can help me

Cheers!

Check MyGUI.log for errors and warnings.