Default installation of latest version, for the tracker Releases, shows a default report with no columns and adding a first column produces this error:

Argument #1 ($var) must be of type Countable Uncaught TypeError: count(): Argument #1 ($var) must be of type Countable|array, null given in /usr/share/tuleap/plugins/tracker/include/Tracker/Report/Tracker_Report_Renderer_Table.class.php:1914

Being PP8.0, there needs to be checks to ensure if the var is null or not an initialized array.

I changed it to this in my installation for a quick fix:

 is_countable($session_table_columns) ? count($session_table_columns) : 0;