Thank you Pankaj,
I updated the T3 Framework plugin from the Joomla panel to the latest version available (3.0.5). Then it started to show the error: Call to a member function get() on null
I decided to update it with the file available from my sensei templete download panel in joomlart (Version 3.0.2) and everything worked correctly already.
Is there any special requirement or I better not update it anymore for now?