Show a user-visible error-message when loading a custom layout failed
A desktop-notification would be a decent way to do that. It should explain why loading the custom layout failed, and preferably also offer to open the relevant custom-layout-file for editing.
This would make it easier for people to design/fix custom layouts.
Squeekboard already logs such error-messages:
Warning: Failed to load layout from Path: "/home/username/.local/share/squeekboard/keyboards/de.yaml": Bad data: YAML: buttons.Shift_L: unknown variant `lockingg`, expected one of `locking`, `set_view`, `show_prefs`, `erase` at line 38 column 15, skipping