machine_name element requires 'exists' entry in ScopeForm

Problem/Motivation

The machine_name field type tries to access a exists element on its render array to ensure the machine_name is unique. However, this value is not provided by the ScopeForm.

Steps to reproduce

1. Ensure error reporting is enabled.
2. Create a new OAuth server
3. Create a new scope

An error will be shown.

Notice: Undefined index: exists in Drupal\Core\Render\Element\MachineName::validateMachineName() (line 266 of core/lib/Drupal/Core/Render/Element/MachineName.php).

Drupal\Core\Render\Element\MachineName::validateMachineName(Array, Object, Array)
call_user_func_array(Array, Array) (Line: 280)

Proposed resolution

Use [$this->storage, 'load'] similar to the other forms.

User interface changes

None

API changes

None

Data model changes

None