exceptions in taurusdesigner (extra_guiqwt)

[sf-migrator-bot]

To reproduce, in debian testing with taurus 4.0.0:

Launch taurusdesigner

Traceback (most recent call last):
  File "/tmp/kkt4/usr/local/lib/python2.7/dist-packages/taurus/qt/qtgui/extra_guiqwt/tools.py", line 264, in items_changed
    self.activate_command(plot, self.action.isChecked())
RuntimeError: wrapped C/C++ object of type QAction has been deleted
Traceback (most recent call last):
  File "/tmp/kkt4/usr/local/lib/python2.7/dist-packages/taurus/qt/qtgui/extra_guiqwt/tools.py", line 264, in items_changed
    self.activate_command(plot, self.action.isChecked())
RuntimeError: wrapped C/C++ object of type QAction has been deleted
TypeError: unable to convert a Python 'NoneType' object to a C++ 'int' instance

Also WARNINGS:

MainThread     WARNING  2016-07-18 17:50:30,503 TaurusRootLogger: Qt: Cannot open file '/home/cpascual/extra_icon:lock_unknown.svg', because: No such file or directory
MainThread     WARNING  2016-07-18 17:50:30,504 TaurusRootLogger: Qt: Cannot open file '/home/cpascual/extra_icon:lock_unknown.svg', because: No such file or directory

Reported by: cpascual ( http://sf.net/u/cpascual )