call to undefinded method rcmail_output_json::button()

[Offerel]

Im a little bit confused. I have now tried the following code in my client.js file:

var menu = rcm_callbackmenu_init({menu_name: 'mynewmenu', menu_source: '#menulist'});
$(buttonid).on("contextmenu", function(e) { rcm_show_menu(e, this, buttonid, mynewmenu); });

buttonid is the id of my button in the taskbar. In my plugin.php file i have a function for adding the menu above the page footer:

public function plugin_submenu()
    {
		$rcmail  = rcmail::get_instance();
			
		$li = '';
		$li .= html::tag('li', null, $rcmail->output->button(array('command' => 'plugin.myplugin.command1', 'type' => 'link', 'class' => 'myclass1', 'label' => 'myplugin.command1')));
		$li .= html::tag('li', null, $rcmail->output->button(array('command' => 'plugin.myplugin.command2', 'type' => 'link', 'class' => 'myclass2', 'label' => 'myplugin.command2')));
		$li .= html::tag('li', null, $rcmail->output->button(array('command' => 'plugin.myplugin.command3', 'type' => 'link', 'class' => 'myclass3', 'label' => 'myplugin.command3')));
		$out = html::tag('ul', array('id' => 'mymenu'), $li);
		$rcmail->output->add_footer(html::div(array('style' => 'display: none;'), $out));
    }

i call the public funnction in the last line of my plugin init with: $this->plugin_submenu();

but i get a error in the roundcube logfile:
PHP Fatal error: call to undefinded method rcmail_output_json::button()

Do you have any idea what i miss or can you please give a little bit more detailed instructions how to implement a submenu to a taskbar button?

Update: I have now seen that the div is added in the html source above the footer and in the header section rcmail.register_button is added for every entry in the menu. However, the php error is not gone :-( I didn't understand why the error occurs and the code is executed.

[Offerel]

Oh my god. I didn't know why, but after changing the init part to

if (!$rcmail->action) {
            $this->plugin_submenu();
        }

instead of only $this->plugin_submenu();, it works Sorry for opening the issue.