NOTICE: Some Mac users see blank rectangles after upgrading to Mac OS 10.13.4.
It doesn't happen to everyone. It didn't happen to me, so I can't study it to find out why.
The good news is that it can be fixed. It needs to be completely uninstalled, then installed again.
To be sure it's completely uninstalled, use the free program called AppCleaner. https://freemacsoft.net/appcleaner/
We'll help the best we can, but please verify that your email
address is entered correctly so that the response reaches you. :)
For Windows 8 or higher, please specify traditional "PC" (legacy), or
Windows "Store" app.