set async=ture when get print/exportButtons

src/main/webapp/resources/js/preview/app.js

@@ -123,7 +123,7 @@ $("body").keydown(function(event) {
 function showExportButtons() {
 	$.ajax({
 		type: "get",
-		async: false,
+		async: true,
 		url: "print/exportButtons",
 		success: function (exportButtons) {
 			if(exportButtons.length){