Run your app in debug mode on simulator or Device and check in the terminal if you see any error releted to "Incorect parentdataWidget" error that can be a potantial issue for the grey screen in release mode.

This error occures from "Expanded" and Flexible widget.

If you encounter "incorrect parentdata widget" in terminal then try to remove expanded widget and add container or other alternative in this screen and try to run in release mode your issue will be fixed.

If you need any more assistance feel free to contact i need some more details to find out the exact problem. Hope this helps.