Window rendering in Yocto beaglebone Black

[RK Suma]

I am building Qt application for beaglebone black board using Yocto project. I have two UI pages, In MainWindow designed icon which is login button by selecting it navigates Dialog page where i have some icons and back button. I have connected My board to Touchscreen and when i boot My Qt application is coming in the startup. then i got to know that, in my application when Dialog page is Displayed over MainWindow if i click anywhere in MainWindow, Dialog page was going behind MainWindow, so i tried below code to resolve it.

dialog = new Dialog(this);
dialog->setWindowFlags(Qt::Dialog | Qt::FramelessWindowHint | 
Qt::MSWindowsFixedSizeDialogHint);

// Set dialog window modality
dialog->setWindowModality(Qt::WindowModal);

Now when i booted this code I am able to login once when i go-back and login once again login button and other functionalities also not working, window is freezed. How to solve this..?