Ok, I found it and it's definitely a bug. The problem starts with enabling "Pause on focus loss" option in the Config-Interface tab.
It looks like there is a new small window appears when a game starts(it wasn't in the previous builds) that get a focus from main window and causing this error.
It looks like there is a new small window appears when a game starts(it wasn't in the previous builds) that get a focus from main window and causing this error.