Observed while building layout-template arguments from the generated JSON schema (toplogic://schema/layout-template/{name}).

Symptom

For the template com.top_logic/dialog.template.xml, the title property is declared non-nullable with an item-default:

<property name="title" type="PolymorphicConfiguration">
    <item-default/>
    <non-nullable/>
</property>

but the generated JSON schema presents it as nullable/optional:

"title": {
    "title": "Title provider",
    "anyOf": [
        { "type": "null" },
        { "$ref": "toplogic://schema/config/com.top_logic.layout.component.title.TitleProvider" }
    ]
}

Because the schema offers {"type": "null"}, a schema-driven client (here: an AI agent authoring the arguments) reads title as optional and omits it.

Consequence

When title is omitted, the non-nullable item-default resolves to a ConstantTitle$Config whose getTitle() is @Mandatory and unset. The resulting component cannot be instantiated:

Property 'com.top_logic.layout.component.title.ConstantTitle$Config.getTitle()': Property is mandatory but not set.

This error is only logged (caught in DatabaseLayoutCacheEntry.put(...), "Cannot parse layout '...'") and the layout silently fails to build — it is not surfaced to the caller.

Expected

The generated schema should reflect the contract:

• A <non-nullable/> property should not include {"type": "null"} in its anyOf (it is not optional).
• For a non-nullable item-default whose concrete default config has @Mandatory nested properties (e.g. ConstantTitle.getTitle()), the schema should surface those nested properties as required, so a client can supply a complete value rather than producing an un-instantiable default.

Notes

• Reproduced against engine 8.0.0-alpha3 via the layout-template config-schema endpoint.
• The same shape likely affects any layout-template property declared non-nullable with an item-default that carries mandatory nested fields.
• Manifested in the tl-ai agentic layout editor, where the agent omitted title and produced an un-instantiable dialog; once the error was reported back (build-time validation with an own protocol), the agent supplied a correct ConstantTitle value.