[taurus4] solve bug NoneType is not iterable on TaurusGUI exit()

In taurus4Taurus exception on clossing Taurus gui with an emtpy Taurus plot:

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtgui/taurusgui/taurusgui.py", line 387, in closeEvent
TaurusMainWindow.closeEvent(self, event)
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtgui/container/taurusmainwindow.py", line 893, in closeEvent
self.saveSettings() # save current window state before closing
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtgui/container/taurusmainwindow.py", line 734, in saveSettings
settings.setValue("TaurusConfig", self.createQConfig())
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtcore/configuration/configuration.py", line 413, in createQConfig
configdict = self.createConfig(allowUnpickable=False)
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtgui/taurusgui/taurusgui.py", line 606, in createConfig
cfg = TaurusMainWindow.createConfig(self, *args, **kwargs)
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtcore/configuration/configuration.py", line 201, in createConfig
itemcfgs[k] = v.createConfig(allowUnpickable=allowUnpickable)
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtgui/taurusgui/taurusgui.py", line 215, in createConfig
configdict['widget'] = self.widget().createConfig()
File "/usr/lib/python2.7/dist-packages/taurus/qt/qtgui/qwt5/taurusplot.py", line 2433, in createConfig
model = CaselessList([m for m in self.getModel() if m in curvenames])
TypeError: 'NoneType' object is not iterable