FIX: update the list of users after user joined group (#13822)

* Make UI update after user have joined a public group

* Check if a group exists in the join method

app/assets/javascripts/discourse/app/models/group.js

@@ -131,7 +131,7 @@ const Group = RestModel.extend({
     return ajax(`/groups/${this.id}/join.json`, {
       type: "PUT",
     }).then(() => {
-      this.findMembers();
+      this.findMembers({}, true);
     });
   },
 

app/controllers/groups_controller.rb

@@ -398,6 +398,7 @@ class GroupsController < ApplicationController
     end
 
     group = Group.find(params[:id])
+    raise Discourse::NotFound unless group
     raise Discourse::InvalidAccess unless group.public_admission
 
     return if group.users.exists?(id: current_user.id)