Clicking "back to parent task" on subtask when parent has been deleted crashes the app

To reproduce:

* create task for today

* create subtask

* drag subtask do tomorrow

* delete parent task

* go to the subtask (still present for tomorrow)

* click on "back to the parent task"

The app will crash.

Expected behavior: no "back to the parent task" link, since the task has been orphaned during the deletion of the parent.