Fix warning message when product variation has no weight

includes/rules_shipping_framework_woocommerce.php

@@ -185,9 +185,11 @@ class RulesShippingFrameworkWooCommerce extends RulesShippingFramework {
 		);
 		foreach ($products as $product) {
 			$w = $product['data']->get_weight();
-			$dimensions['maxweight'] = max ($dimensions['maxweight'], $w);
-			$dimensions['minweight'] = min ($dimensions['minweight'], $w);
-			$dimensions['weight'] += $w * $product['quantity'];
+			if (is_numeric($w)) {
+				$dimensions['maxweight'] = max ($dimensions['maxweight'], $w);
+				$dimensions['minweight'] = min ($dimensions['minweight'], $w);
+				$dimensions['weight'] += $w * $product['quantity'];
+			}
 		}
 		return $dimensions;
 	}