AssertionError when a layout had a block which has been removed

Problem/Motivation

When a layout has a block and that block plugin didn't exist anymore (for example because the module has been uninstalled, the view removed or the custom code deleted), when trying to access to the layout builder with a non-admin user, there would be a fatal error:

AssertionError: assert(isset($build['layout-builder__section'][$region][$uuid]['content'])) in assert() (line 64 of modules/contrib/lb_ux/src/Element/LayoutBuilderUX.php).
assert(, 'assert(isset($build['layout-builder__section'][$region][$uuid]['content']))') (Line: 64)
Drupal\lb_ux\Element\LayoutBuilderUX->buildAdministrativeSection(Object, 1) (Line: 124)
Drupal\layout_builder\Element\LayoutBuilder->layout(Object) (Line: 139)
Drupal\lb_ux\Element\LayoutBuilderUX->layout(Object) (Line: 98)
Drupal\layout_builder\Element\LayoutBuilder->preRender(Array)

Steps to reproduce

- Create a view block
- Place that view on a layout
- Remove that view
- Try to access to the layout builder with a not administrator error.

Proposed resolution

Replace the assert() call with an if statement

Remaining tasks

-
- Review
- Test

User interface changes

None

API changes

None

Data model changes

None