I've encountered this bug a few times now and have sometimes worked around it by closing every other app on my computer:
When I choose to upgrade a graft after a battle, my computer immediately turns black and restarts. Perhaps the graft upgrade screen is using a lot of visual resources? I've narrowed it down to just graft upgrade because I was able to perform every other post-battle reward. Whatever order I do the graft upgrade in, it fails on that particular reward.
Have your computer use a good amount of resources, maybe open youtube or have a remote desktop active as well.
Finish a battle to achieve a graft upgrade.