Elementor Fatal error: Class ‘Elementor\Widget_Base’ not found

I had the same problem. Take $this->includes(); out of the init method, and put it into the register_widgets method:

public function init() {

   ...

   // Include plugin files
   // $this->includes(); // <-- remove this

}

public function register_widgets() {

    $this->includes(); // <- put it here
    \Elementor\Plugin::instance()->widgets_manager->register_widget_type( new \Elementor_oEmbed_Widget() );

}