fix(popup): prevent error if no popups can be displayed
fix(popup): prevent error if no popups can be displayed

If _displayNext was called without an opened popup and without a new
popup to display, an error was thrown.

This also prevent to register the callbacks on
_displayedPopupDefered.promise more than once.

arenaoftitans/arena-of-titans,arenaoftitans/arena-of-titans,arenaoftitans/arena-of-titans