2 deleted 492 characters in body
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Also you dont need to use function beceause there no conditions or values to use inside the function so in yourfull code Null is finechanges :

function _options_page(){
    if($this->args['page_type'] == 'submenu'){
        if(!isset($this->args['page_parent']) || empty($this->args['page_parent'])){
            $this->args['page_parent'] = 'themes.php';
        }
        $this->page = add_theme_page(
                        $this->args['page_parent'],
                        $this->args['page_title'], 
                        $this->args['menu_title'], 
                        $this->args['page_cap'], 
                        $this->args['page_slug'], 
                        array(&$this, '_options_page_html')
                    );
    }else{
        $this->page = add_theme_page(
                        $this->args['page_title'], 
                        $this->args['menu_title'], 
                        $this->args['page_cap'], 
                        $this->args['page_slug'], 
                        array(&$this, '_options_page_html'),
                        $this->args['menu_icon'],
                        $this->args['page_position']
                    );

    if(true === $this->args['allow_sub_menu']){

        //this is needed to remove the top level menu item from showing in the submenu
        add_theme_page(
          $this->args['page_slug'],
          $this->args['page_title'],
          '',
          $this->args['page_cap'],
          $this->args['page_slug'],
          function($a){ return null; }
        );


        foreach($this->sections as $k => $section){

            add_theme_page(
                    $this->args['page_slug'],
                    $section['title'], 
                    $section['title'], 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab='.$k, 
                    function($a){ return null; }
            );

        }

        if(true === $this->args['show_import_export']){

            add_theme_page(
                    $this->args['page_slug'],
                    __('Import / Export', 'nhp-opts'), 
                    __('Import / Export', 'nhp-opts'), 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab=import_export_default', 
                    function($a){ return null; }
            );

        }//if

        foreach($this->extra_tabs as $k => $tab){

            add_theme_page(
                    $this->args['page_slug'],
                    $tab['title'], 
                    $tab['title'], 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab='.$k, 
                    function($a){ return null; }
            );

        }
        if(true === $this->args['dev_mode']){

            add_theme_page(
                    $this->args['page_slug'],
                    __('Dev Mode Info', 'nhp-opts'), 
                    __('Dev Mode Info', 'nhp-opts'), 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab=dev_mode_default', 
                null    function($a){ return null; }
            );

        }//if
    }//if           


    }//else
    add_action('admin_print_styles-'.$this->page, array(&$this, '_enqueue'));
    add_action('load-'.$this->page, array(&$this, '_load_page'));
}//function

Also you dont need to use function beceause there no conditions or values to use inside the function so in your code Null is fine :

       add_theme_page(
                $this->args['page_slug'],
                __('Dev Mode Info', 'nhp-opts'), 
                __('Dev Mode Info', 'nhp-opts'), 
                $this->args['page_cap'], 
                $this->args['page_slug'].'&tab=dev_mode_default', 
                null
        );

full code changes :

function _options_page(){
    if($this->args['page_type'] == 'submenu'){
        if(!isset($this->args['page_parent']) || empty($this->args['page_parent'])){
            $this->args['page_parent'] = 'themes.php';
        }
        $this->page = add_theme_page(
                        $this->args['page_parent'],
                        $this->args['page_title'], 
                        $this->args['menu_title'], 
                        $this->args['page_cap'], 
                        $this->args['page_slug'], 
                        array(&$this, '_options_page_html')
                    );
    }else{
        $this->page = add_theme_page(
                        $this->args['page_title'], 
                        $this->args['menu_title'], 
                        $this->args['page_cap'], 
                        $this->args['page_slug'], 
                        array(&$this, '_options_page_html'),
                        $this->args['menu_icon'],
                        $this->args['page_position']
                    );

    if(true === $this->args['allow_sub_menu']){

        //this is needed to remove the top level menu item from showing in the submenu
        add_theme_page(
          $this->args['page_slug'],
          $this->args['page_title'],
          '',
          $this->args['page_cap'],
          $this->args['page_slug'],
          function($a){ return null; }
        );


        foreach($this->sections as $k => $section){

            add_theme_page(
                    $this->args['page_slug'],
                    $section['title'], 
                    $section['title'], 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab='.$k, 
                    function($a){ return null; }
            );

        }

        if(true === $this->args['show_import_export']){

            add_theme_page(
                    $this->args['page_slug'],
                    __('Import / Export', 'nhp-opts'), 
                    __('Import / Export', 'nhp-opts'), 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab=import_export_default', 
                    function($a){ return null; }
            );

        }//if

        foreach($this->extra_tabs as $k => $tab){

            add_theme_page(
                    $this->args['page_slug'],
                    $tab['title'], 
                    $tab['title'], 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab='.$k, 
                    function($a){ return null; }
            );

        }
        if(true === $this->args['dev_mode']){

            add_theme_page(
                    $this->args['page_slug'],
                    __('Dev Mode Info', 'nhp-opts'), 
                    __('Dev Mode Info', 'nhp-opts'), 
                    $this->args['page_cap'], 
                    $this->args['page_slug'].'&tab=dev_mode_default', 
                    function($a){ return null; }
            );

        }//if
    }//if           


    }//else
    add_action('admin_print_styles-'.$this->page, array(&$this, '_enqueue'));
    add_action('load-'.$this->page, array(&$this, '_load_page'));
}//function
1
source | link

You should be able to change create_function( '$a', "return null;" ) to Anonymous Function (aka Closure) :

From :

    add_theme_page(
            $this->args['page_slug'],
            __('Dev Mode Info', 'nhp-opts'), 
            __('Dev Mode Info', 'nhp-opts'), 
            $this->args['page_cap'], 
            $this->args['page_slug'].'&tab=dev_mode_default', 
            create_function( '$a', "return null;" )
    );

To :

        add_theme_page(
                $this->args['page_slug'],
                __('Dev Mode Info', 'nhp-opts'), 
                __('Dev Mode Info', 'nhp-opts'), 
                $this->args['page_cap'], 
                $this->args['page_slug'].'&tab=dev_mode_default', 
                function($a){ return null; }
        );

Also you dont need to use function beceause there no conditions or values to use inside the function so in your code Null is fine :

       add_theme_page(
                $this->args['page_slug'],
                __('Dev Mode Info', 'nhp-opts'), 
                __('Dev Mode Info', 'nhp-opts'), 
                $this->args['page_cap'], 
                $this->args['page_slug'].'&tab=dev_mode_default', 
                null
        );