could be a system bug? since looks like the infinite "close" send from _NSThemeCloseWidget, no stack of the App is involved...

Thanks, OOPer. I remove the observer in the notification block. Btw: I just saw this crash once and there have call stacks of the system only, also I am not sure when there is -[NSWindow _finishClosingWindow] when launching my App.

Thanks, OOPer. I just do [[NSNotificationCenter defaultCenter] removeObserver:self] inside the block. There are only call stacks of system libraries in the crash log, not my own app stack involved. I only see this crash once. Btw: why I can see NSWindow _finishClosingWindow when launching my App?

[[NSProcessInfo processInfo] operatingSystemVersion] 

Thanks OOPer, I just wan to compare the app path for different OS version: std::vector<Config> gConfigs = { {@APP_PATH, @OSVersion}, ...}; ... In the compare: if (@available(@OSVersion, *) {...} Is there any avaiable API?

Thanks, Eskimo. The testing code is intentional not for singleton, just want to confirm there is no memory leak if I did so:)