I see that someone reported this but did not have instructions on how to reproduce.
Currently using SC 9.0.018, and chrome browser [SIZE=13px]Version 60.0.3112.113 on Windows 10[/SIZE]
Open any project
Click “Project” on the top menu, Click “Generate source code”
Choose “All”
Once finished generating code click on “Close”
Now try closing your project. It will not work you can open other projects but cannot close them. Annoying bug.
Work around - logout and log back in.