[FIX] purchase: don't check subtype if not in the context of purchase order.

Before this commit, we check everytime the subtype.
In some case (eg: portal user on the forum) don't have access to subtype of
type internal.

This commit fixes opw-688403
This commit closes #13510

addons/purchase/purchase.py

@@ -1065,9 +1065,9 @@ class MailComposeMessage(models.TransientModel):
 
     @api.multi
     def send_mail(self, auto_commit=False):
-        compose_internal = self.filtered('subtype_id.internal')
-        if self._context.get('default_model') == 'purchase.order' and self._context.get('default_res_id') and not compose_internal:
-            order = self.env['purchase.order'].browse([self._context['default_res_id']])
-            if order.state == 'draft':
-                order.state = 'sent'
+        if self._context.get('default_model') == 'purchase.order' and self._context.get('default_res_id'):
+            if not self.filtered('subtype_id.internal'):
+                order = self.env['purchase.order'].browse([self._context['default_res_id']])
+                if order.state == 'draft':
+                    order.state = 'sent'
         return super(MailComposeMessage, self.with_context(mail_post_autofollow=True)).send_mail(auto_commit=auto_commit)