Closed Viewfinder?

I have noticed when I swap from viewfinder to live screen, viewfinder get closed so, do I need to put smth on the viewfinder in order to close it when I want to achieve longer exposure when I'm in live screen mode or it's not needed?

Послато са SM-G903F уз помоћ Тапатока

Yes, you should. Whether viewfinder or live view, during the actual long exposure time the position of shutter and mirror are exactly the same, wide open. Live view additionally opens the shutter and lifts the mirror during standby so you can view through the sensor, but that has no affect on what light may leak through during the actual exposure; conditions are identical for the exposure.