Viserion Kills Her
It would be a cruel twist of fate, but Daenerys could meet her end at the claws of one of her own children. With Viserion under the control of the Night King, his mother would have no power over him. And I can't imagine Daenerys being able to stomach the thought of killing her own child. In the thick of battle, she could falter when it comes to attacking her undead child, giving the dragon all the opening he needs to do the deed.