Invalid component trees no longer cause a fatal error when a child cannot be placed in its parent's slot

Problem

ComponentTreeItemList::getHydratedValue() places each child component instance in its parent's slot, which assumes the parent offers that slot. Four scenarios break that assumption:

1. The parent does not exist at all (dangling parent_uuid).
2. The parent failed to hydrate, so it cannot be rendered, and neither can the component instances in its slots.
3. The parent is a kind of component that cannot have slots at all, such as a block component.
4. The parent has slots, but not the one claimed by the child.

Scenarios 1, 3 and 4 are rejected by ComponentTreeStructureConstraintValidator, so they can only reach rendering if validation was bypassed, such as by a custom update path or direct database manipulation. Scenario 2 can also occur for a valid component tree, because hydration depends on state outside it.

Before

Scenarios 1 to 3

A fatal error while rendering the host entity, whether or not assertions were enabled:

array_key_exists(): Argument #2 ($array) must be of type array, null given

Scenario 4

No error in production, where assertions are compiled out: only assertions touched the invalid value. The child was placed in a slot the parent component does not define, and nothing rendered it, because a component's template only prints the slots it declares. With assertions enabled, an assertion failure was raised instead.

After

All scenarios

The unreachable child and all its descendants are discarded, and the rest of the component tree still renders. A parent that failed to hydrate continues to render the usual error container.

Scenarios 1 to 3

A fatal error becomes a rendered page.

Scenario 4

Renders exactly as before: that child produced no output then, and produces none now.

Who is affected

Only sites with invalid component trees, and only as an improvement: a page that returned a fatal error now renders. No content that previously rendered stops rendering. A child that cannot be placed was already invisible, because no slot printed it.

What to do

Run the doctor to find affected data:

vendor/bin/dr canvas:doctor --all-checks --details

Both the config and content checks run the same validation, so every affected component tree is reported. Fix the reported property paths through the editor UI, config import, or a targeted update, then re-run.

No API change

ComponentTreeItemList::getHydratedValue() is private. No public API changed. Sites without invalid component trees are unaffected.