Problem creating an array instance in a widget in 4.4 update

Problem solved! Still don’t know what has changed in the 4.4 update though.

Prior to 4.4, you can use the square brackets reverted to generate an array instance, like that:

<select multiple="multiple" name="<?php echo $this->get_field_name('cats][') ?>">
//$instance['cats'] = array();

Starting from 4.4, you can use square brackets in regular case, like that:

<select multiple="multiple" name="<?php echo $this->get_field_name('cats[]') ?>">
//$instance['cats'] = array();

Or, use the generic and best approach:

<select multiple="multiple" name="<?php echo $this->get_field_name('cats') ?>[]">
//$instance['cats'] = array();