I initially wrote this as a comment on Nij's Answer but thought it deserved its own answer.
It depends on if the Umpire has deemed the ball to be dead
From Law 20 - Dead Ball
20.1.1 The ball becomes dead when
188.8.131.52 it is finally settled in the hands of the wicket-keeper or of the bowler.
20.1.2 The ball shall be considered to be dead when it is clear to the bowler’s end umpire that the fielding side and both batsmen at the wicket have ceased to regard it as in play.
20.2 Ball finally settled
Whether the ball is finally settled or not is a matter for the umpire alone to decide.
So in this case, does the umpire deem the wicketkeeper's action to start throwing back the ball to the bowler as finally settled in the hands of the wicket-keeper, or the fielding side and both batsmen at the wicket have ceased to regard it as in play?
If the umpire still believes the ball to still be in play, then it is Out as highlighted in Nij's answer.
In actual fact the batsman (LJ Evans) in this case was out Run Out, not stumped, as the umpire has deemed the batsman to be attempting a run (probably because he thought the wicket-keeper has missed the ball), and not trying to regain their balance.