A small fix for layouts that refused to save.
Fixes
- Editing a layout now saves reliably, even when the layout file is missing one of its images. Before this, changes to such a layout looked like they went through but never stuck, with no message explaining why.
- When a layout is missing an image, the editor now tells you so ("An image is missing from your template") and drops that empty layer, so the rest of your layout keeps working as it did.