extract named constructor suggestions from prefixed element contributor

pkg/analysis_server/lib/src/protocol_server.dart

Index: pkg/analysis_server/lib/src/protocol_server.dart
diff --git a/pkg/analysis_server/lib/src/protocol_server.dart b/pkg/analysis_server/lib/src/protocol_server.dart
index 678361f5f1b0c397c55b5288345f326de90094ef..5dd4cb9131972ffbb098c6baf2a3bd5563012646 100644
--- a/pkg/analysis_server/lib/src/protocol_server.dart
+++ b/pkg/analysis_server/lib/src/protocol_server.dart
@@ -55,7 +55,7 @@ String getReturnTypeString(engine.Element element) {
     if (element.kind == engine.ElementKind.SETTER) {
       return null;
     } else {
-      return element.returnType.toString();
+      return element.returnType?.toString();
     }
   } else if (element is engine.VariableElement) {
     engine.DartType type = element.type;