Fix a TypeError in CallTipWidget

[rayosborn]

This is a fix for #383. It applies the same modification to the CallTipWidget hideEvent function as that used to fix CompletionWidget in #367.

Fixes #383.

[wmvanvliet]

Yeah, this would work. However, it would be even better if we could prevent the error from occuring in the first place, rather than just swallowing it and pretend it didn't happen.

[rayosborn]

I have no problem with using another solution as long as the fix gets into the next release.

[ccordoba12]

@rayosborn, please merge with master to get the fix to our tests.

[rayosborn]

I'm not sure I understand the request. Should I merge with my fork's master and then reissue the PR?

[ccordoba12]

No, please run these commands instead:

git remote add upstream https://github.com/jupyter/qtconsole.git
git checkout master
git fetch upstream
git merge upstream/master
git checkout fix-calltipwidget
git merge master
git push origin fix-calltipwidget

[ccordoba12]

Thanks @rayosborn for your help!