Error: Unsupported operand types in form_builder() (line 1816 of (...)/includes/form.inc).

After updating core from 7.61 to 7.64 this began to occur on Drupal Commerce checkout:
Error: Unsupported operand types in form_builder() (line 1816 of (...)/includes/form.inc).

I checked the form.inc diff:

And noticed that is_array() validation was removed.

Note that if the $element parameter comes NULL it won't throw anything because setting $element['#processed'] = FALSE; will "cast" it as an array. But, if the $element comes as, for example, an integer, then we'll have a problem.

Unfortunately I wasn't able to retrieve much information from log.

Probably the is_array() validation should be kept.