Trying to access array offset on value of type bool in PHP 7.4

If your query does not return a row, then your variable $Row will be filled with false, so you can test if the variable has a value before try to access any index inside it:

if($Row){
  if(is_null($Row['Data']))
  {
      $session_data = '';
  }...

Easy with PHP ?? null coalescing operator

return $Row['Data'] ?? 'default value';

Or you can use as such

$Row['Data'] ??= 'default value';
return $Row['Data'];