[Users] GridPanel not filled or partially filled

[Spat]

Hi all! 

I am trying to fill a GridPanel with WFS features. But I have the following exception: "uncaught exception: No center region defined in BorderLayout ext-comp-1002".

This exception may happen before WFS request or after.
If it happens before, grid is empty.
If it happens after, grid only contains one line.

Example is here: http://test.vttrack.fr/test/test-geoxt.php

Does anyone encountered such problem or could help me, please?
Thanks a lot!
Nicolas