fixed js error

static/js/script.js

@@ -7,26 +7,35 @@ if (document.body.classList.contains("hero")) {
             document.body.classList.remove("scroll");
     });
 }
-// Sidebar toggle
-document.querySelector('[data-action="toggle-sidebar"]').addEventListener("click", function () {
-    document.body.classList.toggle("sidebar");
-});
-document.querySelector('form').addEventListener('submit', function (e) {
-    var targetUrl = e.target.action;
-    // Load form data
-    var formParameter = GetFormParameter(e.target);
-    // Build url
-    var queryString = Object.keys(formParameter).map(function (k) { return encodeURIComponent(k) + "=" + encodeURIComponent(formParameter[k]); }).join("&");
-    var getUrl = targetUrl + "?" + queryString;
-    // Perform get request
-    var xmlHttp = new XMLHttpRequest();
-    xmlHttp.open("GET", getUrl);
-    xmlHttp.send();
-    // Cleanup
-    e.target.reset();
-    alert("Anmeldung abgeschickt");
-    e.preventDefault();
-});
+window.onload = function () {
+    // Sidebar toggle
+    var sidebarToggleElement = document.querySelector('[data-action="toggle-sidebar"]');
+    if (sidebarToggleElement) {
+        sidebarToggleElement.addEventListener("click", function () {
+            document.body.classList.toggle("sidebar");
+        });
+    }
+    // Form silent submit
+    var formElement = document.querySelector('form');
+    if (formElement) {
+        formElement.addEventListener('submit', function (e) {
+            var targetUrl = e.target.action;
+            // Load form data
+            var formParameter = GetFormParameter(e.target);
+            // Build url
+            var queryString = Object.keys(formParameter).map(function (k) { return encodeURIComponent(k) + "=" + encodeURIComponent(formParameter[k]); }).join("&");
+            var getUrl = targetUrl + "?" + queryString;
+            // Perform get request
+            var xmlHttp = new XMLHttpRequest();
+            xmlHttp.open("GET", getUrl);
+            xmlHttp.send();
+            // Cleanup
+            e.target.reset();
+            alert("Anmeldung abgeschickt");
+            e.preventDefault();
+        });
+    }
+};
 function GetFormParameter(form) {
     var params = new Array();
     var length = form.elements.length;