Add in null check.

src/Compilers/Core/Portable/Syntax/SyntaxTriviaList.cs

@@ -256,8 +256,8 @@ public SyntaxTriviaList InsertRange(int index, IEnumerable<SyntaxTrivia> trivia)
             }
 
             // Just return ourselves if we're not being asked to add anything.
-            var triviaList = trivia as ICollection;
-            if (triviaList.Count == 0)
+            var triviaCollection = trivia as ICollection;
+            if (triviaCollection != null && triviaCollection.Count == 0)
             {
                 return this;
             }