I had this error when upgrading a Drupal 9 installation to PHP8:
Deprecated function: json_decode(): Passing null to parameter #1 ($json) of type string is deprecated in
json_decode($tempstore->get('simulation_ids'))
PHP8 does not accept null values inside json_decode. So make sure it exists:
if ($tempstore->get('simulation_ids')) {
json_decode($tempstore->get('simulation_ids'))
}