Fix NPE in ContextManager

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index 7f4ba1ecf54b7c4458d3bc6d8651d38fba14bc74..6fda06f3bd0e72cbb25a0242378f7d5bb4410354 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -18,6 +18,7 @@ import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:watcher/watcher.dart';
+import 'package:analyzer/src/generated/java_engine.dart';
 
 /**
  * The name of `packages` folders.
@@ -457,8 +458,8 @@ abstract class ContextManager {
    */
   void _destroyContext(Folder folder) {
     _contexts[folder].changeSubscription.cancel();
-    _contexts.remove(folder);
     removeContext(folder);
+    _contexts.remove(folder);
   }
 
   /**