viewsource alert fails when it calls remove alert on DocumentButton

[SAMdroid]

Sometimes the toplevel is changed to DocumentButton, and there is no remove_alert on DocumentButton.

The idea is to check if there is remove_alert on toplevel,
to prevent a false function call

I made this patch because of this error in logs after duplicate activity (appears -I think- 10 seconds after the duplication process)

 Traceback (most recent call last):
File "/home/broot/sugar-build/build/out/install/lib/python2.7/site-packages/jarabe/view/viewsource.py", line 470, in __alert_response_cb self.get_toplevel().remove_alert(alert)
AttributeError: 'DocumentButton' object has no attribute 'remove_alert'

​https://github.com/sugarlabs/sugar/pull/582

Ignacio and I can't find a method to reproduce it yet