Add check for null reply_to_user

lib/pleroma/web/templates/static_fe/static_fe/_notice.html.eex

@@ -46,9 +46,11 @@
           <%= gettext("Reply to") %>
         </a>
         <span class="h-card">
+          <%= if @reply_to_user do %>
             <a href="<%= (@reply_to_user.uri || @reply_to_user.ap_id) %>" class="u-url mention">
               @<%= @reply_to_user.nickname %>
             </a>
+          <% end %>
         </span>
       </div>
       <% end %>