Fix 404 on group page if user is not member of page

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/layouts/nav/_group.html.haml

@@ -12,11 +12,11 @@
           = link_to projects_group_path(@group), title: 'Projects' do
             Projects
         %li.divider
-        - if @group && can?(current_user, :admin_group, @group)
+        - if can?(current_user, :admin_group, @group)
           %li
             = link_to edit_group_path(@group) do
               Edit Group
-        - if access = @group.users.find(current_user.id)
+        - if access = @group.users.find_by(id: current_user.id)
           %li
             = link_to leave_group_group_members_path(@group),
               data: { confirm: leave_group_message(@group.name) }, method: :delete, title: 'Leave group' do