Fix crash for watch of partial function on Revit

- Fixed null pointer exception for watch in RevitWatchHandler
- For partial function, it now shows _SingleFunctionObject
- Fixed
http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2522
http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2495
http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2264