when the screen turns off viber closes

my m2 has 4.5.1u and i have problem with viber.when the screen turns off after a while viber closes.
i have check the security center so the problem it is not there.does anyone has the same problem?

open taskbar/ hold your app and wait for the lock to arise. This should fix it.

There are some settings in security->battery which sets which apps will not be killed in stand—by mode.

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