Problem with remove_tag() method - AttributeError: 'NoneType' object has no attribute 'get_children'

[Tom Kadwill]

Whist implementing a delete tag feature (for bug #1044639) I encountered a problem with the remove_tag() method. The issue does not cause a problem with GTG in terms of functionality but shows an error when running from the debug script.

To reproduce the error use the remove_tag() method to remove a tag (or hit the delete key if my solution for bug #1044639 has been merged). Then open a task a hit backspace, you should then see an error and associated stack trace in the terminal. I have pasted mine below as an example:

Traceback (most recent call last):
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/gtk/editor/taskview.py", line 586, in modified
    self.save_task()
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/gtk/editor/editor.py", line 513, in light_save
    self.save()
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/gtk/editor/editor.py", line 497, in save
    self.task.sync()
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/core/task.py", line 612, in sync
    self.modified()
  File "/usr/lib/python2.7/dist-packages/liblarch/treenode.py", line 51, in modified
    self.tree.modify_node(self.node_id, priority=priority)
  File "/usr/lib/python2.7/dist-packages/liblarch/tree.py", line 88, in modify_node
    self._queue.push(self._modify_node, node_id, priority=priority)
  File "/usr/lib/python2.7/dist-packages/liblarch/processqueue.py", line 65, in push
    func(*element[1:])
  File "/usr/lib/python2.7/dist-packages/liblarch/tree.py", line 269, in _modify_node
    self._callback('node-modified', node_id)
  File "/usr/lib/python2.7/dist-packages/liblarch/tree.py", line 78, in _callback
    func(node_id)
  File "/usr/lib/python2.7/dist-packages/liblarch/viewcount.py", line 85, in __modify
    displayed &= filt.is_displayed(nid)
  File "/usr/lib/python2.7/dist-packages/liblarch/filters_bank.py", line 42, in is_displayed
    value = self.func(task, parameters=self.dic)
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/core/treefactory.py", line 146, in tag_filter
    return node.has_tags([tag])
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/core/task.py", line 767, in has_tags
    toreturn = children_tag(tagname)
  File "/home/tom/bazaar/gtg/delete-tag2/GTG/core/task.py", line 751, in children_tag
    for tagc_name in tag.get_children():
AttributeError: 'NoneType' object has no attribute 'get_children'