Avoid closing an already closed channel

pkg/analysis_server/lib/src/plugin/plugin_manager.dart

Index: pkg/analysis_server/lib/src/plugin/plugin_manager.dart
diff --git a/pkg/analysis_server/lib/src/plugin/plugin_manager.dart b/pkg/analysis_server/lib/src/plugin/plugin_manager.dart
index 11b61997ce1b3e41b264889385044dcde171c1ef..b83c042bd1432c6f9d0798d4e3a58030d2861e86 100644
--- a/pkg/analysis_server/lib/src/plugin/plugin_manager.dart
+++ b/pkg/analysis_server/lib/src/plugin/plugin_manager.dart
@@ -892,8 +892,10 @@ class PluginSession {
    * Handle the fact that the plugin has stopped.
    */
   void handleOnDone() {
-    channel.close();
-    channel = null;
+    if (channel != null) {
+      channel.close();
+      channel = null;
+    }
     pluginStoppedCompleter.complete(null);
   }