If your Vivi is frozen with with a screen that says "Select your screen from the pop-up window to start sharing", the quickest solution is to log out of the Vivi app and then log back in.

To do so, open the Vivi app on your device, and navigate to the gear icon in the top right

From the settings page, scroll down slightly until you see the "Log Out" button with the power icon next to it and click it.

This will return you to the log in screen on the app, where you can enter your email and password again to re-log in. Upon logging in the projector should become unfrozen and allow you to present normally.