Comment 10 for bug 1253735

Same Error on Ubuntu 18.04
I am adding this comment because this error is "not exclusive" to Fedora 20 and may happen on any Linux distro.

Traceback (most recent call last):
  File "/usr/share/gtg/GTG/gtk/plugins.py", line 254, in on_plugin_toggle
    self.pengine.activate_plugins([plugin])
  File "/usr/share/gtg/GTG/core/plugins/engine.py", line 221, in activate_plugins
    plugin.instance.activate(api)
  File "/usr/share/gtg/GTG/plugins/urgency_color/urgency_color.py", line 45, in activate
    self._refresh_task_color()
  File "/usr/share/gtg/GTG/plugins/urgency_color/urgency_color.py", line 48, in _refresh_task_color
    self._plugin_api.set_bgcolor_func(self.bgcolor)
  File "/usr/share/gtg/GTG/core/plugins/api.py", line 216, in set_bgcolor_func
    for pane in browser.vtree_panes.itervalues():
AttributeError: TaskEditor instance has no attribute 'vtree_panes'

Only occurs when a task window is open.
    Freezes program, had to close and re-open
    When re-opened, the plug-in was no longer checked.
As long as only main window is open this error does not occur.

Workaround in Ubuntu 18.04 is:
Ensure only main window is open before adding a plugin.