“undefined function is_plugin_active()” error report and fix howto

I get the following error message:
PHP Fatal error: Uncaught Error: Call to undefined function is_plugin_active()

Here’s how to fix it:

/**
 * check other plugin
 */
function gf_cgfm_check_other_plugin() {

   /**
    * Ensure is_plugin_active() exists (not on frontend)
    */
   if ( ! function_exists( 'is_plugin_active' ) ) {
      include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
   }

   /**
    * Detect plugin WooCommerce Mailerlite
    */
   if ( ! is_plugin_active( 'woo-mailerlite/woo-mailerlite.php' ) ) {
      // Plugin is activated.
      require 'vendor/autoload.php';
   }
}

add_action( 'plugins_loaded', 'gf_cgfm_check_other_plugin' );

You need to put the check in the plugins_loaded hook. And load the is_plugin_active function from core.