NullReferenceException in ExpressionEvaluator.EvaluateWorker

[MikhailArkhipov]

From microsoft/vscode-python#2297

[Error - 07:34:56] System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Object.GetType()
   at Microsoft.PythonTools.Analysis.Analyzer.ExpressionEvaluator.EvaluateWorker(Node node)
   at Microsoft.PythonTools.Analysis.Analyzer.DDG.Walk(ClassDefinition node)
   at Microsoft.PythonTools.Parsing.Ast.ClassDefinition.Walk(PythonWalker walker)
   at Microsoft.PythonTools.Analysis.Analyzer.DDG.Walk(SuiteStatement node)
   at Microsoft.PythonTools.Parsing.Ast.SuiteStatement.Walk(PythonWalker walker)
   at Microsoft.PythonTools.Analysis.Analyzer.DDG.WalkBody(Node node, AnalysisUnit unit)
   at Microsoft.PythonTools.Analysis.ClassAnalysisUnit.AnalyzeWorker(DDG ddg, CancellationToken cancel)
   at Microsoft.PythonTools.Analysis.Analyzer.DDG.Analyze(Deque`1 queue, CancellationToken cancel, Action`1 reportQueueSize, Int32 reportQueueInterval)
   at Microsoft.PythonTools.Analysis.PythonAnalyzer.AnalyzeQueuedEntries(CancellationToken cancel)
   at Microsoft.PythonTools.Intellisense.AnalysisQueue.GroupAnalysis.Analyze(CancellationToken cancel)
   at Microsoft.PythonTools.Intellisense.AnalysisQueue.Worker(Object threadStarted)