Editor: removing non-existing subtask

[Izidor Matušov]

Steps to reproduce:
1) Have task structure:
A
-> B
2) Open task A's editor
3) Delete task B in the taskbrowser
4) Select line with task B in A's editor and remove it (press delete)

Expected result:
Subtask B is removed or even better - task A window is updated after made change

Actual result:
Traceback:
Traceback (most recent call last):
  File "/usr/share/gtg/GTG/gtk/editor/taskview.py", line 729, in _delete_range
    self.remove_subtask(target)
  File "/usr/share/gtg/GTG/core/task.py", line 403, in remove_child
    c.remove_parent(self.get_id())
AttributeError: 'NoneType' object has no attribute 'remove_parent'

Version: rev.1038

Remarks: This should be changed during editor rewrite.