Fix error related to weakref if creating a viewer failed

[astrofrog]

This fixes the following exception:

Traceback (most recent call last):
  File "/Users/tom/Dropbox/Code/Glue/glue/glue/app/qt/mdi_area.py", line 61, in dropEvent
    new_layer(layer)
  File "/Users/tom/Dropbox/Code/Glue/glue/glue/app/qt/mdi_area.py", line 51, in new_layer
    self._application.choose_new_data_viewer(layer)
  File "/Users/tom/Dropbox/Code/Glue/glue/glue/app/qt/application.py", line 849, in choose_new_data_viewer
    return self.do(cmd)
  File "/Users/tom/Dropbox/Code/Glue/glue/glue/core/application_base.py", line 218, in do
    return self._cmds.do(command)
  File "/Users/tom/Dropbox/Code/Glue/glue/glue/core/command.py", line 127, in do
    result = cmd.do(self._session)
  File "/Users/tom/Dropbox/Code/Glue/glue/glue/core/command.py", line 218, in do
    self.created = weakref.ref(v)
TypeError: cannot create weak reference to 'NoneType' object

if e.g. trying to initialize a 3D volume viewer with a 1D dataset, which causes a GUI error and makes add_data return False.