Handle ids of OO read only users correctly

www/common/onlyoffice/inner.js

@@ -919,7 +919,10 @@ define([
         };
 
         const getMyOOIndex = function() {
-            return findUserByOOId(myOOId).index;
+            const user = findUserByOOId(myOOId);
+            return user
+                ? user.index
+                : content.ids.length; // Assign an unused id to read-only users
         };
 
         var getParticipants = function () {