Error: Call to a member function moduleExists() on null in Drupal\field_validation\Form\FieldValidationRuleFormBase->buildForm()

The error is coming from the FieldValidationRuleAddForm class, it extends FieldValidationRuleFormBase but it doesn't call the base class' constructor. As a result, the injected services in the base class aren't being set up.

One solution is to create the child classes with a superset of all the services, then have each child class call the parent class constructor with the arguments it expects.

Patch on request.

1, This issue related 📌 \Drupal calls should be avoided in classes, use dependency injection instead Fixed , the patch looks good, and I commit it directly 2 days ago.
2, TolstoyDotCom is right, I will fixed it later.
3, another solution, you can change the code back:

from

    if ($this->moduleHandler->moduleExists('token')) {

to

 // Add a token link.
    if (\Drupal::moduleHandler()->moduleExists('token')) {

Fixed.

Automatically closed - issue fixed for 2 weeks with no activity.