Update project_invoice.py

odex25_project/project_base/models/project_invoice.py

@@ -233,8 +233,9 @@ class ProjectInvoice(models.Model):
 
     def action_set_to_draft(self):
         self.ensure_one()
-        if self.invoice_id.state=='draft':
-            self.invoice_id.unlink()
+        if self.invoice_id and self.invoice_id.state=='draft':
+            self.invoice_id.sudo().write({'posted_before':False})
+            self.invoice_id.sudo().unlink()
             self.state = 'draft'
         else:
             raise UserError(_("Kindly The invoice is not in draft state, so it cannot be unlinked."))