gtg crashed with AttributeError in checkTomboyPresent(): pluginTomboy instance has no attribute 'plugin_api'

Steps to reproduce:
1, Make sure the system doesn't have Tomboy/Gnote
2, Start GTG, Enable Tomboy Plugin
3, Open a new task. Because the system doesn't have any Tomboy/Gnote, GTG attempts to show an error message (No Tomboy/Gnote). Plugin_api is not set and the traceback is raised.

<traceback>
Traceback (most recent call last):
  File "/usr/share/gtg/GTG/gtk/browser/browser.py", line 1039, in on_add_task
    self.vmanager.open_task(uid,thisisnew=True)
  File "/usr/share/gtg/GTG/gtk/manager.py", line 177, in open_task
    clipboard = self.clipboard)
  File "/usr/share/gtg/GTG/gtk/editor/editor.py", line 160, in __init__
    self.pengine.onTaskLoad(self.plugin_api)
  File "/usr/share/gtg/GTG/core/plugins/engine.py", line 252, in onTaskLoad
    plugin.instance.onTaskOpened(plugin_api)
  File "/usr/share/gtg/GTG/plugins/tomboy/tomboy.py", line 162, in onTaskOpened
    if not self.checkTomboyPresent() or not plugin_api.is_editor():
  File "/usr/share/gtg/GTG/plugins/tomboy/tomboy.py", line 75, in checkTomboyPresent
    self.plugin_api.get_ui().get_window(),
AttributeError: pluginTomboy instance has no attribute 'plugin_api'
</traceback>

Reason:
self.plugin_api is initialized after calling self.checkTomboyPresent(). However, it needs self.plugin_api for showing the error message.