Prevent exception with isolate failed to start

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

Index: pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
diff --git a/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart b/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
index 3b90b51e4c9425947016d09baabae300dde31d56..c0f65fb5487faf2bb4e0c984bea78e11dd6d08e6 100644
--- a/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
+++ b/pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart
@@ -280,10 +280,12 @@ abstract class ServerIsolateChannel implements ServerCommunicationChannel {
 
   @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);
+    }
   }
 
   /**