When the session attributes don't exist, $data->sessionAttributes->getRawValue() cannot be called as not being defined.
So check the session attributes before calling getRawValue()
Old notation: bundle:section:name.format:renderer (where both format and renderer are optional)
New notation: bundle:section:name.format.renderer (where only format is optional)
Valid new template names: Blog:Post:index.php, Blog:Post:index.xml.php
The new notation is more explicit and put all templating engines on the same level (there is no
more the concept of a "default" templating engine).
Even if the notation changed, the semantic has not. So, the logical template name for the above
examples is still 'index'. So, if you use a database loader for instance, the template
name is 'index' and everything else are options.
Upgrading current applications can be easily done by appending .php to each existing template
name reference (in both controllers and templates), and changing :twig to .twig for Twig templates
(for twig templates, you should also add .twig within templates themselves when referencing
another Twig templates).
