Uncaught Error: Call to a member function is_in_stock() on null

if ( $product->is_in_stock() ) {

add_filter( 'woocommerce_get_availability', 'dispay_custom_icons_for_availability', 1, 2);
function dispay_custom_icons_for_availability( $availability, $product ) {
   global $product;

   // available
    if ( $product->is_in_stock() ) {
        $availability['availability'] = '<i class="fas fa-layer-group" style="display:block;color:green;font-size:17px;"> In Stock!</i>';
        $availability['class'] = 'in_stock';
    }

    // middle stock
    if ( $product->is_in_stock() && $product->get_stock_quantity() <= $product->get_low_stock_amount() ) {
        $availability['availability'] = '<i class="fas fa-layer-group" style="display:block;color:orange;font-size:17px;"> Half left</i>';
        $availability['class'] = 'low_stock';
 }


   // out of stock
    if ( ! $product->is_in_stock() ) {
        $availability['availability'] = '<i class="fas fa-layer-group" style="display:block;color:red;font-size:17px;"> Sold out</i>';
        $availability['class'] = 'out_of_stock';
    }

    return $availability;
}