Form element validation #element_validate not working I am porting this contrib module to D8 and need some help.

The module is shipped with a custom node type. Certain elements options in the node_form will be populated dynamically. Also other modules needs to able to provide options for these fields.

hook_form_FORM_ID_alter calls the wrapper class MyModuleManager that performs several helper duties. All is going well except that I can not get the custom element_validation to work.

Upon form validation a warning is generated

Warning: call_user_func_array() expects parameter 1 to be a valid callback, class 'FilebrowserManager' not found in Drupal\Core\Form\FormValidator-&gt;doValidateForm() (line 282 of core/lib/Drupal/Core/Form/FormValidator.php).


together with the fatal error:

Recoverable fatal error: Argument 1 passed to Drupal\Core\Field\WidgetBase::massageFormValues() must be of the type array, string given, called in /var/www/drupal8/core/lib/Drupal/Core/Field/WidgetBase.php on line 376 and defined in Drupal\Core\Field\WidgetBase-&gt;massageFormValues() (line 511 of core/lib/Drupal/Core/Field/WidgetBase.php).


The fatal error occurs exactly when validating $form['my_field']

class MyModuleManager implements MyModuleManagerInterface {
  use StringTranslationTrait;

  public function addFormElements(array $form, FormStateInterface $form_state, NodeInterface $node, AccountInterface $account, $collapsed = TRUE) {
    ...

    return $form;
  }

  public function addFormDefaultOptions(array $form, FormStateInterface $form_state) {

    // Replace MyModule_hook by Events
    // ...
    // Events returns dynamic $options

    $form['my_field']['widget']['#options'] = $options;
    $form['my_field']['widget']['#element_validate'] = array(array($this, 'myModuleValidateTrue()'));

    return $form;
  }

  public function myModuleValidateTrue() {
    return;
  }

}


As the warning tells it can not find the callback in FormValidator I suspect the problem might be the way I wrapped my code in this class. Being new to OOP I would appreciate all help.
It's probably this line:

$form['my_field']['widget']['#element_validate'] = array(array($this, 'myModuleValidateTrue()'));


Which should be this, without parentheses in the method name:

$form['my_field']['widget']['#element_validate'] = array(array($this, 'myModuleValidateTrue'));


See PHP: Callbacks / Callables for the full story.