Emailing an invoice generates Network error

With this extension enabled, when I view a contribution and click to Email the invoice, this generates a "Network Error" alert. Looking in the browser console I see a 500 error with the URL https://mydomain.com/wp-admin/admin.php?page=CiviCRM&q=civicrm%2Fcontribute%2Finvoice%2Femail&reset=1&id=191&cid=109&select=email&snippet=json&crmAngularModules=crmResource

In the apache error log I see:

Got error 'PHP message: PHP Fatal error:  Declaration of CRM_Invoicehelper_Contribute_Form_Task_Invoice::buildQuickForm() must be compatible with CRM_Contribute_Form_Task_Invoice::buildQuickForm(): void in /home/wp/sites/wp/wp-content/uploads/civicrm/ext/invoicehelper-2.0.0/CRM/Invoicehelper/Contribute/Form/Task/Invoice.php on line 9', referer: https://mydomain.com/wp-admin/admin.php?page=CiviCRM&q=civicrm%2Fcontribute&reset=1

I had the exact same issue earlier using CiviCRM 5.72 and version 1.2 of this extension.

I upgraded to CiviCRM 5.73.4 and version 2.0.0 of this extension. Still generates the same error.

Disabling this extension enables me to send the invoice.