Prevent exception with isolate failed to start

pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:isolate';
 
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer_plugin/channel/channel.dart';
@@ skipping 262 matching lines @@
           instrumentationService.logPluginNotification(pluginId, encodedInput);
           onNotification(new Notification.fromJson(input));
         }
       }
     });
     return channelReady.future;
   }
 
   @override
   void sendRequest(Request request) {
-    Map<String, Object> json = request.toJson();
-    String encodedRequest = JSON.encode(json);
-    instrumentationService.logPluginRequest(pluginId, encodedRequest);
-    _sendPort.send(json);
+    if (_sendPort != null) {
+      Map<String, Object> json = request.toJson();
+      String encodedRequest = JSON.encode(json);
+      instrumentationService.logPluginRequest(pluginId, encodedRequest);
+      _sendPort.send(json);
+    }
   }
 
   /**
    * Spawn the isolate in which the plugin is running.
    */
   Future<Isolate> _spawnIsolate();
 }