Lord Of The Light.
In season 7 episode 1 the red priestess convinces Dany to summon Jon to dragonstone because she saw it in the flames.
If Jon had never come to dragonstone, wight napping adventure wouldn't have happened and nor Dany would have to arrive at Eastwatch with her dragons.
Hence no wight dragon.
This is reinforced by the fact that the hound sees a mountain in the flames in s7e1, hence in wight napping episode he leads them there in that setting so they could be trapped and Dany has to come with her dragons.
Either Lord of the Light is evil or the dead have to march south for them to be defeated?