DynamicTheme is not initialized in sessions that did not serve index.html

[javier-godoy]

DynamicTheme stores its state in a VaadinSession attribute that is only set by DynamicTheme.initialize(...), which runs while index.html is being generated (from AppShellConfigurator.configurePage or an IndexHtmlRequestListener). If a UI runs in a session that never served index.html, the attribute is missing and:

• DynamicTheme.prepare(...) and DynamicTheme.apply(...) throw IllegalStateException: Dynamic theme switching has not been initialized
• DynamicTheme.getCurrent() returns null
• TabbedDemo hides the theme selector without any message, because it checks isFeatureInitialized()

This happens in production, not in local development. Two situations cause it:

1. The server restarts or scales to zero while a page is open. The session is lost, and the next request from the open tab runs in a new session that never went through index.html.
2. index.html is served from a cache (browser, proxy or CDN), so configurePage / the index HTML listener never runs for the session that serves the UI.

Stack trace (commons-demo 5.5.0-SNAPSHOT, Vaadin 25.3.0)

java.lang.IllegalStateException: Dynamic theme switching has not been initialized
    at com.flowingcode.vaadin.addons.demo.DynamicTheme.assertFeatureInitialized(DynamicTheme.java:87)
    at com.flowingcode.vaadin.addons.demo.DynamicTheme.prepare(DynamicTheme.java:213)
    at ...(attach listener calling DynamicTheme.prepare(menuBar))
    at com.vaadin.flow.component.ComponentUtil.onComponentAttach(ComponentUtil.java:316)
    ...
    at com.vaadin.flow.component.UI.browserNavigate(UI.java:2205)
    ...
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocations(...)

Steps to reproduce

1. Call DynamicTheme.LUMO.initialize(settings) in configurePage, and call DynamicTheme.prepare(component) on a component in the view.
2. Open the app in production mode.
3. Restart the server without reloading the browser tab. (Alternatively, serve index.html from a cache.)
4. Interact with or navigate in the open tab.

Expected: the view works with the default theme.
Actual: IllegalStateException from prepare (or a null from getCurrent()).

Suggested fix

Don't make "initialized" depend on the index.html request of the current session. For example:

• initialize(...) could also save the chosen default somewhere that is not tied to one session (such as the VaadinContext, or a static field set when DynamicThemeInitializer runs), and getCurrent() could fall back to that default when the session attribute is missing, storing it in the session at that point.
• Alternatively, DynamicThemeInitializer could register a SessionInitListener that sets the default theme for every new session.

Either way, prepare/apply would no longer throw in these situations. The page may still have a different theme stylesheet than the session thinks (for example, aura.css from an earlier session). That is harmless, because apply() enables and disables the <link> elements on the page regardless of which one is currently active.

Workaround

In the application, register a SessionInitListener that sets DynamicTheme.LUMO (or whichever theme is the default) as the session attribute when it is missing.