check for null completion node

pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart

 // Copyright (c) 2014, 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.
 
 library services.completion.dart;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/completion/arglist_computer.dart';
@@ skipping 100 matching lines @@
    * Compute suggestions based upon cached information only
    * then send an initial response to the client.
    * Return a list of computers for which [computeFull] should be called
    */
   List<DartCompletionComputer> computeFast(DartCompletionRequest request) {
     return request.performance.logElapseTime('computeFast', () {
       CompilationUnit unit = context.parseCompilationUnit(source);
       request.unit = unit;
       request.node = new NodeLocator.con1(request.offset).searchWithin(unit);
       request.target = new CompletionTarget.forOffset(unit, request.offset);
+      if (request.node == null) {
+        return [];
+      }
 
       request.replacementOffset = request.offset;
       request.replacementLength = 0;
       var entity = request.target.entity;
       Token token = entity is AstNode ? entity.beginToken : entity;
       if (token != null &&
           token.offset <= request.offset &&
           (token.type == TokenType.KEYWORD || token.type == TokenType.IDENTIFIER)) {
         request.replacementOffset = token.offset;
         request.replacementLength = token.length;
@@ skipping 202 matching lines @@
    * Information about the types of suggestions that should be included.
    * The [target] must be set first.
    */
   OpType get optype {
     if (_optype == null) {
       _optype = new OpType.forCompletion(target, offset);
     }
     return _optype;
   }
 }