Sometimes, `UIActivityViewController` will dismiss the parent view
controller on completion. We have a hack to get around that: present an
invisible VC behind the share sheet. When the share sheet is done, we
dismiss the invisible VC.
However, if you share to an app that crashes (which is out of our
control), we won't always dismiss the invisible VC. I don't know why
this is, but double-dismissing VC works.