[core] Fix error of saving copied document

Internal Server Error: /oozie/editor/workflow/save/
Traceback (most recent call last):
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/core/handlers/base.py", line 249, in _legacy_get_response
    response = self._get_response(request)
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/utils/decorators.py", line 185, in inner
    return func(*args, **kwargs)
  File "/Users/fbpi/projects/hadoop_stack/hue/apps/oozie/src/oozie/decorators.py", line 115, in decorate
    return view_func(request, *args, **kwargs)
  File "/Users/fbpi/projects/hadoop_stack/hue/apps/oozie/src/oozie/decorators.py", line 105, in decorate
    return view_func(request, *args, **kwargs)
  File "/Users/fbpi/projects/hadoop_stack/hue/apps/oozie/src/oozie/views/editor2.py", line 207, in save_workflow
    workflow_doc = _save_workflow(workflow, layout, request.user, fs=request.fs)
  File "/Users/fbpi/projects/hadoop_stack/hue/apps/oozie/src/oozie/models2.py", line 3788, in _save_workflow
    workflow_doc1 = workflow_doc.doc.get()
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/fbpi/projects/hadoop_stack/hue/build/env/lib/python2.7/site-packages/Django-1.11.20-py2.7.egg/django/db/models/query.py", line 380, in get
    self.model._meta.object_name
DoesNotExist: Document matching query does not exist.