I recently deleted the default email template for invoice emails, and now when I click on the 'Send by Email' button, an error is thrown:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/8.0/openerp/http.py", line 537, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/odoo/src/odoo/8.0/openerp/http.py", line 574, in dispatch
    result = self._call_function(**self.params)
  File "/home/odoo/src/odoo/8.0/openerp/http.py", line 310, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/odoo/src/odoo/8.0/openerp/service/model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/src/odoo/8.0/openerp/http.py", line 307, in checked_call
    return self.endpoint(*a, **kw)
  File "/home/odoo/src/odoo/8.0/openerp/http.py", line 803, in __call__
    return self.method(*args, **kw)
  File "/home/odoo/src/odoo/8.0/openerp/http.py", line 403, in response_wrap
    response = f(*args, **kw)
  File "/home/odoo/src/odoo/8.0/addons/web/controllers/main.py", line 948, in call_button
    action = self._call_kw(model, method, args, {})
  File "/home/odoo/src/odoo/8.0/addons/web/controllers/main.py", line 936, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/home/odoo/src/odoo/8.0/openerp/api.py", line 268, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/src/odoo/8.0/addons/portal_sale/portal_sale.py", line 109, in action_invoice_sent
    action_dict = super(account_invoice, self).action_invoice_sent(cr, uid, ids, context=context)
  File "/home/odoo/src/odoo/8.0/openerp/api.py", line 268, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/src/odoo/8.0/openerp/api.py", line 399, in old_api
    result = method(recs, *args, **kwargs)
  File "/home/odoo/src/odoo/8.0/addons/account/account_invoice.py", line 397, in action_invoice_sent
    default_template_id=template.id,
AttributeError: 'NoneType' object has no attribute 'id'

 Can someone please explain to me, in very precise steps, how I can set a different template as the default.