Gets the form state for this form.
\Drupal\views\ViewStorageInterface $view: The view being edited.
string|null $display_id: The display ID being edited, or NULL to load the first available display.
string $js: If this is an AJAX form, it will be the string 'ajax'. Otherwise, it will be 'nojs'. This determines the response.
array An associative array containing the current state of the form.
public function getFormState(ViewStorageInterface $view, $display_id, $js);