Check if form mode is present before trying to extract field_group form information

Problem/Motivation

Working with commerce, I'm getting this error when going to add a product and a store has not been created yet:

( ! ) Fatal error: Call to a member function getMode() on a non-object in /my/project/web/modules/contrib/field_group/field_group.module on line 120

Problem here is that ProductForm class is overriding ContentEntityForm::buildForm() and bypassing the parent implementation under some circumstances.
Thing is that field_group_form_alter() is assuming that $form_state->getStorage()['form_display'] is always present, but this is not always true, as in this case.

Proposed resolution

Add a check to trigger the field_group process only if the form mode is explicitly declared

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None