These days I suppose lot of Linux users face problem running Unetbootin blank program window.
Looks like some bug in a program and there is (I suppose) for now this solution:
Just make new shortcut in your Ubuntu or other Linux distribution (Applications-System tools-Preferences-Main menu for Ubuntu 18.04) with following command:
sudo QT_X11_NO_MITSHM=1 unetbootin
Also choose to run in terminal window so you can type your sudo password.
As far I can remember looks like it is somehow connected with latest kernel distribution of Ubuntu 18.04.2 LTS so there should be soon new update for this problem.
73 de Z34PEC