WordPress / Gravity Forms：使用文本字段时出错并用税填充它

I am using the following Plugin to let users post their content with Gravity Forms: https://wordpress.org/support/plugin/gravity-forms-custom-post-types

When I using a textfield and want to save taxonomies in it, I get the following error (frontend and backend):

Fatal error: Cannot use string offset as an array in ..../gravity-forms-custom-post-types/gfcptaddonbase.php on line 301

Any idea why?

This is the function which causes the problem:

function setup_taxonomy_field( &$field, $taxonomy ) {

        $first_choice = $field['choices'][0]['text'];
        $field['choices'] = $this->load_taxonomy_choices( $taxonomy, $field['type'], $first_choice );

        //now check if we are dealing with a checkbox list and do some extra magic
        if ( $field['type'] == 'checkbox' ) {
            //clear the inputs first
            $field['inputs'] = array();

            $counter = 0;
            //recreate the inputs so they are captured correctly on form submission
            foreach( $field['choices'] as $choice ) {
                $counter++;
                if ( ($counter % 10) == 0 ) $counter++; //thanks to Peter Schuster for the help on this fix
                $id = floatval( $field['id'] . '.' . $counter );
                $field['inputs'][] = array('label' => $choice['text'], 'id' => $id);
            }
        }
    }

If I comment out the first line of the function, it causes no error anymore, and the tags are submitted anyway. But do I need this line?