Move assertion into GetDocument() method

src/EditorFeatures/Core/Implementation/IntelliSense/Completion/Controller_TypeChar.cs

@@ -264,12 +264,14 @@ private static bool IsPotentialFilterCharacter(TypeCharCommandArgs args)
 
         private Document GetDocument()
         {
-            return this.SubjectBuffer.CurrentSnapshot.GetOpenDocumentInCurrentContextWithChanges();
+            // Crash if we don't find a document, we're already in a bad state.
+            var document = this.SubjectBuffer.CurrentSnapshot.GetOpenDocumentInCurrentContextWithChanges();
+            Contract.ThrowIfNull(document, nameof(document));
+            return document;
         }
 
         private CompletionHelper GetCompletionHelper()
         {
-            // Crash if we don't find a document, we're already in a bad state.
             var document = GetDocument();
             return CompletionHelper.GetHelper(document);
         }