Thank you @andypiper. I am urgently releasing a fix to the library but this is already affecting multiple users.
I know this has nothing to do with you but the dev team should inform the developers about this type of change. Right now multiple application break as this change was not prepared and the FavouriteCount property have always been a non nullable int since I started to work with the API.
For any Tweetinvi developer coming here please download the new version that has just been released on nuget 1.2.0.1. nuget.org/packages/TweetinviAPI/