Importing a layout page causes "missing callback" error and "Undefined array key" warning

[argiepiano]

Description of the bug

Importing a custom layout page created on a different site causes "missing callback" error and an "Undefined array key..." Warning.

The imported page layout actually shows up under "Layout overrides" instead of "Layout Pages"

Steps To Reproduce

To reproduce the behavior:

1. Create a layout page in Site A
2. In the dropbutton for "manage layout" select "export layout"
3. Copy the json code
4. Paste in in Site B's Configuration Manager > Single Import and save
5. Visit the Structure > Layouts UI and see the problem

Actual behavior

Layout page is imported as a non-functional Layout override, a "missing callback" error is shown, and a warning is shown.

Expected behavior

The Layout page would be imported and it would work as expected.

Additional information

I believe the issue here is that a Layout Page has TWO config files; one is the layout, and the other the layout menu item. For my example pasted above, these are:

• layout.layout.shopping_cart.json
• layout.menu_item.shopping_cart.json

So, you need to export/import BOTH files to Site B in order for the imported layout to actually work. The problem happens when you are unaware of this, and you only import the layout json. I assume most users will trust that the "Export layout" link in the dropbutton will give them a "working version" that can be imported in another site, but that's not the case. That only gives you one of the json files.

I really can't think what a good solution might be. I guess we could automatically generate the menu item json upon import (but this will create problems if the person has customized that menu item), or at least throw a warning explaining what's going on.

• Backdrop CMS version: 1.28.0
• PHP version: 8.1

[klonos]

In order to address this properly, we need config dependencies, which is not currently a thing in Backdrop. We basically need #4188.