Game crashes once the phone enters lock screen mode.

Hey, I wanted to know why my game crashes when the phone enters standby mode(?) (The mode in which you have to swipe the screen to unlock the phone).
In the appDelegate class I do have these three functions:

virtual bool applicationDidFinishLaunching();

virtual void applicationDidEnterBackground();

virtual void applicationWillEnterForeground();

Should there be another function for when the mobile locks screen?

Thanks for your time!