Calls DefinitionProvider.get() too eagerly:

• first, AppDescriptorRegistry tries to get the descriptor to fire AppRegistryEvent with; but only the app name is used. We can surely live firing the event with the related DefinitionMetadata instead.
• second, AppLauncherLayoutManagerImpl#getLayoutForCurrentUser() iterates over groups and apps, doesn't check if provider is valid before adding it.

Side note for future plans:

• we should most likely remove the AppLauncherLayoutManager altogether and move the "current user layout" logic to the shell app (and spare one useless event "conversion" too).
• RegistryEvents would probably make sense on the AbstractRegistry level now.