Only update_and_save if the stack exists

The following error can happen during a stack-delete:

  AttributeError: 'NoneType' object has no attribute 'update_and_save'

Only update_and_save the stack if it is not None.

Change-Id: I8c27ed6a461c699eba7da5bef8e879f1c0bdaf6b
Closes-Bug: #1274664
Merge "Only update_and_save if the stack exists"