checkLazySingletonInstanceExists<T extends Object> abstract method

Checks if a lazy singleton instance has been created and exists.

This method verifies whether a lazy singleton of type T (optionally identified by instanceName) has been instantiated. Lazy singletons are only created when first accessed via get.

Returns true if the instance has been created and exists, false if it hasn't been created yet.

Throws a StateError if the type T is not registered as a lazy singleton in GetIt.

Example:

// Register a lazy singleton
getIt.registerLazySingleton<MyService>(() => MyService());

// Check if instance exists (will return false initially)
bool exists = getIt.checkLazySingletonInstanceExists<MyService>();

// Access the instance (creates it)
MyService service = getIt<MyService>();

// Check again (will return true now)
exists = getIt.checkLazySingletonInstanceExists<MyService>();