Add null check before returning postfix expansion

pkg/analysis_server/lib/src/edit/edit_domain.dart

Index: pkg/analysis_server/lib/src/edit/edit_domain.dart
diff --git a/pkg/analysis_server/lib/src/edit/edit_domain.dart b/pkg/analysis_server/lib/src/edit/edit_domain.dart
index 82056becb3f70010647287d55caf83a355c1d177..d60d677649e0f8c13437e97d2a65fadf493a88a1 100644
--- a/pkg/analysis_server/lib/src/edit/edit_domain.dart
+++ b/pkg/analysis_server/lib/src/edit/edit_domain.dart
@@ -290,7 +290,7 @@ class EditDomainHandler extends AbstractRequestHandler {
         PostfixCompletionProcessor processor =
             new PostfixCompletionProcessor(context);
         PostfixCompletion completion = await processor.compute();
-        change = completion.change;
+        change = completion?.change;
       }
     }
     if (change == null) {