FormAPI was made a single-instance plugin so that the event handler only gets called once. Otherwise it would not have been approved. Nevertheless, developers must add the "depend: [FormAPI]" line in the plugin.yml
Because it can not be used for incorporation for the reasons mentioned above, line 33 of Main.php is called PHP: $this->FormAPI = new FormAPI(); not, PHP: $this->FormAPI = $this->getServer()->getPluginManager()->getPlugin("FormAPI"); And you have to. Pull request will be sent so please refer to that for details.