You need to grant permissions to TST Indent Lines, to access information in private (incognito) windows with two steps:
1) "Add-ons Manager" => "Extensions" => "TST Indent Line" => "Details" => "Run in Private Windows" => "Allow"
2) "Add-ons Manager" => "Extensions" => "Tree Style Tab" => "Options" => "Extra Features via Other Addons" => "Permissions for API Call from Other Addons" => turn on the checkbox in the column "Notify Messages from Private Windows" for the row "tst-indent-line@piro.sakura.ne.jp"