After reparenting children via drag and drop, doing "Open Parent" twice causes a `KeyError` traceback, can lead to data file corruption from base_store

[nekohayo]

Possibly a closely related issue to #1018, but the symptom and reproduction steps seems different.
However the steps are very consistent and easy to reproduce, so this might be quite useful for diagnosis by @diegogangl or anyone else.

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With git master:

1. rm -R tmp/default/ && ./launch.sh; this will launch with the clean default dataset (i.e. demo/tutorial tasks)
2. Unfold the main task
3. Drag one of the child tasks (ex: "Learn About Plugins") to become a child of another (ex: "Reporting Bugs")
4. Double-click "Learn About Plugins" (now a child of "Reporting Bugs")
5. Click "Open Parent" once, everything is fine
6. Click "Open Parent" a second time to go up one more level

Result: Global generic exception below.

What's worse is that this apparently(?) corrupts the data file; on subsequent startup, if you open "Reporting Bugs" and click to "Open Parent" only once, you will get the same traceback.

Traceback (most recent call last):
  File "GTG/gtk/application.py", line 444, in open_parent_task
    editor.open_parent()
  File "GTG/gtk/editor/editor.py", line 805, in open_parent
    self.app.open_task(parent)
  File "GTG/gtk/application.py", line 558, in open_task
    editor = TaskEditor(app=self, task=task)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "GTG/gtk/editor/editor.py", line 192, in __init__
    self.textview.insert(text)
  File "GTG/gtk/editor/taskview.py", line 788, in insert
    self.process()
  File "GTG/gtk/editor/taskview.py", line 228, in process
    self.delete_subtask_cb(tid)
  File "GTG/gtk/editor/editor.py", line 762, in remove_subtask
    self.app.ds.tasks.unparent(tid, self.task.id)
  File "GTG/core/tasks.py", line 936, in unparent
    super().unparent(item_id, parent_id)
  File "GTG/core/base_store.py", line 165, in unparent
    raise KeyError
KeyError