test: fix test & implementation error with missing buttons (ids) in UI

js/privatebin.js

@@ -4326,6 +4326,10 @@ window.PrivateBin = (function () {
          * @function
          */
         me.hideEmailButton = function () {
+            if (!emailLink) {
+                return;
+            }
+
             emailLink.classList.add('hidden');
             emailLink.removeEventListener('click', sendEmail);
         }

js/test/TopNav.js

@@ -20,7 +20,8 @@ describe('TopNav', function () {
                     <span class="glyphicon glyphicon-duplicate" aria-hidden="true"></span> Clone</button>
                     <button id="rawtextbutton" type="button" class="hidden btn btn-warning navbar-btn">
                     <span class="glyphicon glyphicon-text-background" aria-hidden="true"></span> Raw text</button>
-                    <button id="qrcodelink" type="button" data-toggle="modal" data-target="#qrcodemodal" class="hidden btn btn-warning navbar-btn">
+                    <button id="downloadtextbutton" type="button" class="hidden btn btn-<?php echo $isDark ? 'warning' : 'default'; ?> navbar-btn"></button>
+                    <button id="qrcodelink" type="button" data-toggle="modal" data-target="#qrcodemodal" class="hidden btn btn-warning navbar-btn"/>
                     <span class="glyphicon glyphicon-qrcode" aria-hidden="true"></span> QR code</button></li></ul></div>
                     </nav>`
                 );