Fix mini bugs js formsemestre_validation_but. Closes #1035 #1036.

app/static/js/jury_but.js

@@ -62,7 +62,7 @@ $(function () {
         next_idx = cur_idx + 1;
       }
     }
-    if (prev_idx != -1) {
+    if ((prev_idx != -1) && (etudids[prev_idx] != null)) {
       let elem = document.querySelector("div.prev a");
       if (elem) {
         elem.href = elem.href.replace("PREV", etudids[prev_idx]);
@@ -71,7 +71,7 @@ $(function () {
     } else {
       document.querySelector("div.prev").innerHTML = "";
     }
-    if (next_idx != -1) {
+    if ((next_idx != -1) && (etudids[next_idx] != null)) {
       let elem = document.querySelector("div.next a");
       if (elem) {
         elem.href = elem.href.replace("NEXT", etudids[next_idx]);
@@ -114,9 +114,10 @@ function get_form_state() {
 
 $("document").ready(function () {
   FORM_STATE = get_form_state();
-  document
-    .querySelector("form#jury_but")
-    .addEventListener("submit", jury_form_submit);
+  let the_form = document.querySelector("form#jury_but");
+  if (the_form) {
+    the_form.addEventListener("submit", jury_form_submit);
+  }
 });
 
 function is_modified() {