Dvc: Exception is raised when adding the same data again

[shcheklein]

DVC version: 0.24.3

Steps to reproduce:

echo 'test' > foo
dvc add foo
dvc add foo

Output:
```Debug: UPDATE state_info SET count = 13 WHERE rowid = 1

Error: unexpected error - 'NoneType' object has no attribute 'dump'

Traceback (most recent call last):
File "/Users/ivan/Projects/cookiecutter-data-science/dvc/.venv/lib/python2.7/site-packages/dvc/main.py", line 30, in main
ret = cmd.run_cmd()
File "/Users/ivan/Projects/cookiecutter-data-science/dvc/.venv/lib/python2.7/site-packages/dvc/command/base.py", line 50, in run_cmd
return self.run()
File "/Users/ivan/Projects/cookiecutter-data-science/dvc/.venv/lib/python2.7/site-packages/dvc/command/add.py", line 10, in run
self.project.add(target, recursive=self.args.recursive)
File "/Users/ivan/Projects/cookiecutter-data-science/dvc/.venv/lib/python2.7/site-packages/dvc/project.py", line 226, in add
stage.dump()

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

Having any troubles? Hit us up at https://dvc.org/support, we are always happy to help!
```

[anotherbugmaster]

Hi guys, just encountered the same issue.

[efiop]

Hi @anotherbugmaster !

Fix is ready https://github.com/iterative/dvc/pull/1583 . We'll merge and release it ASAP.

Thanks for the feedback :slightly_smiling_face:

[anotherbugmaster]

:rocket: