When the card Progenitus is milled by the card Monster Manual, Progenitus goes back into the deck.
However, Monster Manual still believes there is a milled creature to be selected, and will grey out the “done” button when Progenitus is the only creature milled this way.

With no other options, the game freezes, forcing you to surrender or lose due to time out, as there are no more creatures to select.

Bug found on IOS.

“Done” should let you click it if there are no creatures to select, and shouldn’t count Progenitus if it is milled back into the deck.