"Could not contact the server" when indecisively editing a post

[happy-river]

To reproduce:

• Make a post
• Click on "edit" under the post title
• Click on the struckthrough "edit" link to make the edit box go away
• Click on "edit" again
• Actually edit it this time
• Click on "Save Changes"

Result: a request is made to /do/edit_txtpost/null which returns a 500