Fix a null pointer exception

pkg/analysis_server/lib/src/computer/computer_hover.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 computer.hover;
 
 import 'package:analysis_server/src/protocol.dart' show HoverInformation;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 
@@ skipping 50 matching lines @@
   final CompilationUnit _unit;
   final int _offset;
 
   DartUnitHoverComputer(this._unit, this._offset);
 
   /**
    * Returns the computed hover, maybe `null`.
    */
   HoverInformation compute() {
     AstNode node = new NodeLocator.con1(_offset).searchWithin(_unit);
+    if (node == null) {
+      return null;
+    }
     if (node.parent is TypeName &&
         node.parent.parent is ConstructorName &&
         node.parent.parent.parent is InstanceCreationExpression) {
       node = node.parent.parent.parent;
     }
     if (node.parent is ConstructorName &&
         node.parent.parent is InstanceCreationExpression) {
       node = node.parent.parent;
     }
     if (node is Expression) {
@@ skipping 31 matching lines @@
       hover.propagatedType = _safeToString(expression.propagatedType);
       // done
       return hover;
     }
     // not an expression
     return null;
   }
 
   static _safeToString(obj) => obj != null ? obj.toString() : null;
 }