How to undo 'Do not ask again' when exiting?

Enable the Confirm application exit option:

@Serge Baranov  thanks

Not working for me. It still closes the project without asking.

PyCharm 2019.1.2

The project is closed without asking, this option is to confirm application exit.

Got it. Thanks.

Since 2020.2 it has been renamed to "Preferences>Appearance&Behavior>System Settings>Confirm before exiting IDE"

I am wondering if there is any way to ask confirmation before I close a project, since I find that closing it accidentally and reopening it trigger time-consuming rebuilding.

Shenbo There is no such feature yet. 

Feel free to create a feature request at https://youtrack.jetbrains.com/newIssue

An issue has been created here: https://youtrack.jetbrains.com/issue/IDEA-277160?_ga=2.35596065.1644694390.1630381161-599761042.1572851529