Can't change picture immediately after creating a tree with a picture (but no other data)

[RickMohr]

1. Add a tree, supplying a picture but no other fields
2. Without deselecting, click at the base of the map to go to the tree details
3. Click "Edit", supply a different picture, and click "Done" to save the update

The save fails, and server exception is "Can't update id attribute". Debugger shows that tree.id is None.

It works fine if:

• you supply other fields initially, or
• you don't supply a picture initially, or
• you deselect and reselect the tree