changeset.js: do not lose sync in the timeslider if another user deletes text

If a user deleted text/attributes while another one had the timeslider open,
the timeslider lost sync and spit out errors.

Fixes #3932.

src/static/js/Changeset.js

@@ -1529,7 +1529,17 @@ exports.moveOpsToNewPool = function (cs, oldPool, newPool) {
   return upToDollar.replace(/\*([0-9a-z]+)/g, function (_, a) {
     var oldNum = exports.parseNum(a);
     var pair = oldPool.getAttrib(oldNum);
-    if(!pair) exports.error('Can\'t copy unknown attrib (reference attrib string to non-existant pool entry). Inconsistent attrib state!');
+
+    /*
+     * Setting an empty pair. Required for when delete pad contents / attributes
+     * while another user has the timeslider open.
+     *
+     * Fixes https://github.com/ether/etherpad-lite/issues/3932
+     */
+    if (!pair) {
+      pair = [];
+    }
+
     var newNum = newPool.putAttrib(pair);
     return '*' + exports.numToString(newNum);
   }) + fromDollar;