This was fixed in the patch:
https://community.alteryx.com/t5/Alteryx-Designer-Desktop-Discussions/Union-Tool-The-Input-quot-2-quot-was-not-initialized/td-p/1005274/page/2
But if you want a quick fix, try this:
https://community.alteryx.com/t5/Alteryx-Designer-Desktop-Discussions/Input-was-not-initialized/td-p/704626