error due to php 7.1.5

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How to solve this

[09-Oct-2018 12:43:18 Europe/Berlin] PHP Warning: A non-numeric value encountered in ../includes/classes/categories_ul_generator.php on line 58

line 58 is this code

$result = sprintf($this->parent_group_start_string, ($submenu==true) ? ' class="level'. ($level+1) . '"' : '' );

on printing $this->parent_group_start_string echos blank

In another server where php is 5.6 , doesn't generate the error

Add "$level = (int)$level;" so it look like this,

        $level = (int)$level;
        $result = sprintf($this->parent_group_start_string, ($submenu==true) ? ' class="level'. ($level+1) . '"' : '' );

or use the "/includes/classes/categories_ul_generator.php" file from the Zen Cart 1.5.6 repo on github