Fix error in retrieving dialogue poll definition.

Error log:
```
013-08-15 10:01:29+0000 [-] Starting sandbox ...
2013-08-15 10:01:29+0000 [-] Loading sandboxed code ...
2013-08-15 10:01:29+0000 [-] 
2013-08-15 10:01:29+0000 [-] undefined:31
2013-08-15 10:01:29+0000 [-]                 self.start_state = poll.start_state
2013-08-15 10:01:29+0000 [-]                                        ^
2013-08-15 10:01:29+0000 [-] TypeError: Cannot read property 'start_state' of null
```