Prevent crashing website

Applying the latest release causes some websites (not all) to show 'the
website encountered an unexpected error' with no further information. The
watchdog shows:
php       RuntimeException: An invalid implementation
cern_integration_form_alter was added by hook_module_implements_alter() in
Drupal\Core\Extension\ModuleHandler->buildImplementationInfo() (line
error     https://dev-d8-rwmanos.web.cern.ch/

Although this additional if condition is not in the documentation example
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Extension%21module.api.php/function/hook_module_implements_alter/8.5.x

An example implementing it uses it, for instance,
https://api.drupal.org/api/drupal/core%21modules%21system%21tests%21modules%21common_test%21common_test.module/8.5.x

cern_integration.module

@@ -76,7 +76,7 @@ function cern_integration_form_user_register_form_alter(&$form, &$form_state, $f
 * As this module injects a checkbox on user_form, cern_integration module must be unstacked.
 */
 function cern_integration_module_implements_alter(&$implementations, $hook) {
-  if ($hook == 'form_alter') {
+  if ($hook == 'form_alter' && isset($implementations['cern_integration'])) {
     // Move my_module_form_alter() to the end of the list.
     // \Drupal::moduleHandler()->getImplementations()
     // iterates through $implementations with a foreach loop which PHP iterates