Hi!

After plugin activation, there is a Fatal error

Fatal error: Uncaught Error: Call to undefined function is_plugin_active() in plugins/edd-invoiced/includes/class-edd-invoiced-handler.php:399 Stack trace: #0 plugins/edd-invoiced/includes/class-edd-invoiced-handler.php(26):Edd_Invoiced_Handler->hooks() #1 plugins/edd-invoiced/includes/class-edd-invoiced-plugin.php(27): Edd_Invoiced_Handler::get_instance() #2 plugins/edd-invoiced/edd-invoiced-plugin.php(34): Edd_Invoiced_Plugin::get_instance() #3 plugins/edd-invoiced/edd-invoiced-plugin.php(63): EddInvoicePlugin() #4 wp-settings.php(362)….

PHP 7.3
WP 5.3

I installed the plugin, after activating I received a white screen error 500. I disconnected all the plugins, then I switched on the EDD and your plugin, again the error 500.