Indeed it's a small bug. You have to edit the file widget.php in administrator/components/com_hikashop/classes and replace these lines:
foreach($elements as $element){
if(!isset($typeTable[$element->type])){
$typeTable[$element->type]=$element->type;
}
}
by these:
if(!empty($elements)){
foreach($elements as $element){
if(!isset($typeTable[$element->type])){
$typeTable[$element->type]=$element->type;
}
}
}
By the way, filtering by product on that kind of report is useless since the line of customers just show the number of new customers. Those who just registered.
So filter by product is meaningless.