removed the returnValue property from the saveInterface class as it i…

…s deprecated and the preventDefault property is sufficient (#3379)

js/SaveInterface.js

@@ -75,7 +75,7 @@ class SaveInterface {
 
         this.timeLastSaved = -100;
         const $j = jQuery.noConflict();
-        $j(window).bind("beforeunload", (event) => {
+        $j(window).on("beforeunload", (event) => {
             let saveButton = "#saveButtonAdvanced";
             if (this.activity.beginnerMode) {
                 saveButton = "#saveButton";
@@ -86,7 +86,6 @@ class SaveInterface {
                 this.PlanetInterface !== undefined
             ) {
                 event.preventDefault();
-                event.returnValue = "";
                 // Will trigger when exit/reload cancelled.
                 $j(saveButton).trigger("mouseenter");
                 return "";