Unable to show window on WINCE

Steps to reproduce / test case
1. Write a simple program with one form based on QMainWindow. The window is maximized by calling QWidget::showMaximized().
For instance, the program in the Qt example(QTDIR\examples\mainwindows\application)
2. Compile and deploy it on WinCE
3. Start the program.
4. If there is not a cross button at the top right corner. Start another program in WinCE. Then click the cross button to close it.
5. There should be a cross button at the top right corner. Click the button.
6. Activate the program from the task manager or reclick the program from Windows Explorer.
7. The title and menu of the program will appear but the content of the window is not updated.
More information
Clicking the cross button at the top right corner does not close the program but minimize the window on WinCE platform. It means the program cannot be restored on the platform. I've tried to add something in the showEvent but the showEvent is not activated when the program is activated from the task manager.
(QTDIR\examples\mainwindows\application) is tested on my device and is suitable to reproduce the case.