Fix exception on loading File Manager.

chrome/browser/resources/file_manager/js/file_manager.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Setting the src of an img to an empty string can crash the browser, so we
 // use an empty 1x1 gif instead.
 
 /**
  * FileManager constructor.
  *
@@ skipping 1134 matching lines @@
                (this.dialogType_ == FileManager.DialogType.SELECT_SAVEAS_FILE ||
                 this.dialogType_ == FileManager.DialogType.FULL_PAGE);
 
       case 'unmount':
         return true;
 
       case 'format':
         var entry =
             this.getRootEntry_(this.rootsList_.selectionModel.selectedIndex);
 
-        return DirectoryModel.getRootType(entry.fullPath) ==
-               DirectoryModel.RootType.REMOVABLE;
+        return entry && DirectoryModel.getRootType(entry.fullPath) ==
+                        DirectoryModel.RootType.REMOVABLE;
     }
   };
 
   FileManager.prototype.getRootEntry_ = function(index) {
     if (index == -1)
       return null;
 
     return this.rootsList_.dataModel.item(index);
   };
 
@@ skipping 2982 matching lines @@
           });
       }, onError);
 
       function onError(err) {
         console.log('Error while checking free space: ' + err);
         setTimeout(doCheck, 1000 * 60);
       }
     }
   }
 })();