Fix null default value access (#29512).

pkg/analysis_server/lib/src/services/completion/dart/utilities.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/utilities.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/utilities.dart b/pkg/analysis_server/lib/src/services/completion/dart/utilities.dart
index 314cbf4bfa76bf1f17f4ae5b0caaf16a1f1b521c..bd4421d4a59342e33507e58304d00bd886ccb992 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/utilities.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/utilities.dart
@@ -76,7 +76,7 @@ void addDefaultArgDetails(
       if (isFlutterWidget(element.enclosingElement)) {
         for (ParameterElement param in element.parameters) {
           if (param.name == 'children') {
-            String defaultValue = getDefaultStringParameterValue(param);
+            String defaultValue = getDefaultStringParameterValue(param) ?? '';

[Brian Wilkerson, 2017/04/28 18:20:26]

Or just fix `getDefaultStringParameterValue` to never return `null`?

[pquitslund, 2017/04/28 18:24:25]

We use `null` as a sentinel elsewhere (in arglist_contributor) where it's useful
to distinguish none from an empty string.

Might be worth consolidating approaches though...

I'll take a look a this as I'll be in the area working on Flutter `children:`
completions (https://github.com/flutter/flutter-intellij/issues/463).

             if (sb.isNotEmpty) {
               sb.write(', ');
             }