fix: assign dialog instance to a variable in update_child_items function

`dialog` is being referred in `onchange` handlers of the fields. Without this fix, they are failing because `dialog` is not defined.

erpnext/public/js/utils.js

@@ -750,7 +750,7 @@ erpnext.utils.update_child_items = function (opts) {
 		});
 	}
 
-	new frappe.ui.Dialog({
+	let dialog = new frappe.ui.Dialog({
 		title: __("Update Items"),
 		size: "extra-large",
 		fields: [
@@ -787,7 +787,9 @@ erpnext.utils.update_child_items = function (opts) {
 			refresh_field("items");
 		},
 		primary_action_label: __("Update"),
-	}).show();
+	});
+	
+	dialog.show();
 };
 
 erpnext.utils.map_current_doc = function (opts) {