Hi Abrar! DataGrid2 has issues with rendering widgets inside itself.
Try these steps:
1) You can open browser's developer tool and check if your popup menu is present in the DOM.
2) If it is available, try updating the CSS to match your needs. If it is not available, then you need to investigate on why it was not loaded. Checking errors in console will help.
3) If your CSS works, Move it to your Mendix app and redeploy.