Show scan does not validate door

[cmft]

Showscan reports attribute error if the door name does not exist.

    load_entry_point('sardana==3.0.4a0', 'gui_scripts', 'showscan')()
  File "/usr/lib/python3/dist-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python3/dist-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python3/dist-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/sardana/taurus/qt/qtgui/extra_sardana/showscanonline.py", line 318, in main
    widget.setModel(door)
  File "/usr/lib/python3/dist-packages/sardana/taurus/qt/qtgui/extra_sardana/showscanonline.py", line 238, in setModel
    self.info_form.setModel(model)
  File "/usr/lib/python3/dist-packages/sardana/taurus/qt/qtgui/extra_sardana/showscanonline.py", line 113, in setModel
    door.recordDataUpdated.connect(self.onRecordDataUpdated)
AttributeError: 'NoneType' object has no attribute 'recordDataUpdated'