After doing some initial investigations I found on line number 209 of the cla...

After doing some initial investigations I found on line number 209 of the class Drupal\Core\Form\FormBuilder

the first index of the `$args` array is unset to remove the argument passed to the metod `getForm`. But in case of Druapl checkout flow the value of the first index of `$args` is an instance of `MultistepDefault` plugin which actully get unset hence above error is thrown.