Configure plugin button remains clickable upon reopening the preferences menu and makes GTG crash if clicked

[Roquentin]

Steps to reproduce:
1. Open preferences
2. Make "Configure plugin" button senstive by selecting a plugin.
3. Close preferences dialog
4. Open preferences
5. Click on the button "Configure plugin" without selecting any plugin.

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I have enabled a configurable GTG plugin in the preferences and I have modified some options in the corresponding menu. Then I close the preferences menu and I reopen it. I go to Plugins again and now the "Configure Plugin" button is still clickable (it is not greyed out) even though no plugin is selected. If I accidentally click on the "Configure Plugin" button, GTG crashes with the following message

Traceback (most recent call last):
  File "/usr/share/gtg/GTG/taskbrowser/preferences.py", line 282, in on_plugin_configure
    plugin_id = self.plugin_store.get_value(iter, PLUGINS_COL_ID)
TypeError: iter must be a GtkTreeIter

When using version 0.2.4, GTG prints the message but keeps running (the exception is caught somewhere?). Same problem on trunk (rev 1181), but here GTG crashes right away.

I experienced it when playing around with the color urgency plugin in trunk, but this is reproducible on different configurable plugins.

[Izidor Matušov (izidor)]

Fixed in rev. 1184.