Issue title: Natron crashes as soon as you key something in the roto node
 Issue body: ### Problem

Natron crashes as soon as you hit "set key" on any parameter of the roto node. The autokeying of the shape works without problems.
### Steps to Reproduce
1. create a roto-node
2. create a roto shape
3. try to keyframe any parameter
### Versions
- **Natron version/commit** (they can be retrieved from the About Window): 
  tags/2.0.0 at version ca2c00bc5393b8c37197746e9a858d76cd969034
- **OS version:** 
  Windows 7

 Comments: 
 Comment 0: Was fixed in https://github.com/MrKepzie/Natron/commit/9c4bcd2f5fb1f755bf5dffb8c488731711008e3f
{'source': 'GithubIssues', 'repo_name_with_owner': 'MrKepzie/Natron', 'issue_id': 141560067, 'repo_stars': 1470.0}