Labels disappear after moving files to a folder or after an undo action

Ubuntu 11.04 (64-bit), Dutch language
Openshot 1.4.0-beta1 (latest trunk, revision 574)

Steps to reproduce:
1. Start OpenShot
2. Import several clips
3. Label some or all of the clips
4. Remove the last imported clip from the "Project files" tree
5. Press "Undo"

Result: all the labels of all the clips disappear

6. Label some or all of the clips again
7. Create one or more folders
8. Label the folder(s)
9. Move a clip to a folder

Result: labels of all the folders disappear

Same happens when you import a clip after labeling a folder

Traceback (most recent call last):
  File "/home/joop/Development/openshot/openshot/windows/TreeFiles.py", line 143, in cell_edited
    self.project.project_folder.UpdateFileLabel(unique_id, new_text, 0)
  File "/home/joop/Development/openshot/openshot/classes/files.py", line 507, in UpdateFileLabel
    if item.unique_id == unique_id:
AttributeError: OpenShotFolder instance has no attribute 'unique_id'