fixed js error

src/typescript/script.ts

@@ -8,33 +8,40 @@ if (document.body.classList.contains("hero")) {
     });
 }
 
-// Sidebar toggle
-document.querySelector('[data-action="toggle-sidebar"]').addEventListener("click",() => {
-	document.body.classList.toggle("sidebar");
-});
-
-document.querySelector('form').addEventListener('submit', (e : any) => {
-    let targetUrl = e.target.action;
-
-    // Load form data
-    let formParameter : Array<string> = GetFormParameter(e.target);
-
-    // Build url
-    let queryString = Object.keys(formParameter).map((k : any) => encodeURIComponent(k) + "=" + encodeURIComponent(formParameter[k])).join("&");
-    let 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",() => {
+            document.body.classList.toggle("sidebar");
+        });
+    }
 
+    // Form silent submit
+    var formElement = document.querySelector('form');
+    if(formElement) {
+        formElement.addEventListener('submit', (e : any) => {
+            let targetUrl = e.target.action;
+        
+            // Load form data
+            let formParameter : Array<string> = GetFormParameter(e.target);
+        
+            // Build url
+            let queryString = Object.keys(formParameter).map((k : any) => encodeURIComponent(k) + "=" + encodeURIComponent(formParameter[k])).join("&");
+            let 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 : any )
 {