// Test with fallback interface and invalid plugin_id.
$factory_ref->setValue($manager, $this->getMockFactoryInterface(2));
$fallback_result = $manager->createInstance('invalid', $configuration_array);
$this->assertEquals('invalid_fallback', $fallback_result['plugin_id']);
// Test with fallback interface and invalid plugin_id.
$factory_ref->setValue($manager, $this->getMockFactoryInterface(2));
$fallback_result = $manager->createInstance('invalid', $configuration_array);
$this->assertEquals('invalid_fallback', $fallback_result['plugin_id']);