Deleting a category without a parent sets an invalid parent on its subcategories

When a category whose parent is empty is deleted from Blog.ManageCategories, its subcategories get a parent that is the literal string

{Blog.Categories.WebHome}

(with curly braces) instead of the categories root, so they no longer appear in the categories tree.

Cause: #deleteCategory in Blog.CategoriesCode builds the fallback parent with

"{$defaultCategoryParent}"

instead of

"${defaultCategoryParent}"

.