Modified getUserName function

src/core/js/caosdb.js

@@ -53,9 +53,15 @@ function isAuthenticated() {
  * @return Name of the user.
  */
 function getUserName() {
-    return document.getElementsByClassName("caosdb-user-name")[0].innerText;
+    const userElement = document.getElementsByClassName("caosdb-user-name")[0];
+    if (userElement) {
+        return userElement.innerText;
+    } else {
+        throw new Error("No user is logged in.");
+    }
 }
 
+
 /**
  * Return the realm of the user currently logged in.
  *