Fixed: requesting the single test case print popup (lib/testcases/tcPrint.php) with a valid testcase_id while the session carries no project ($_SESSION['testprojectID'] unset) and the URL has no tproject_id parameter produced a broken page: HTTP 500 with a raw debug_print_backtrace() dump instead of the printable test case.

This is the fresh-session deep-link scenario: e.g. a bookmark or an external tool linking straight to

/lib/testcases/tcPrint.php?testcase_id=1002&tcversion_id=2002

The response body was only:

<pre>#0 .../lib/functions/print.inc.php(2326): testproject->get_by_id(0)
#1 .../lib/functions/print.inc.php(951): initStaticRenderTestCaseForPrinting(...)
#2 .../lib/testcases/tcPrint.php(64): renderTestCaseForPrinting(...)
</pre>

plus HTTP status 500. No printable content at all.

• tcPrint.php::init_args() resolves the project id from $_SESSION['testprojectID'], then falls back to the tproject_id request parameter. In the deep-link scenario both are absent, so $args->tproject_id stays 0.
• The rendering pipeline passes that 0 down as $context['tproject_id']; initStaticRenderTestCaseForPrinting() (lib/functions/print.inc.php) called testproject::get_by_id($tprojectID) unconditionally.
• testproject::get_by_id() treats a falsy id as fatal: if (null == $id) — and under PHP 8 the loose comparison null == 0 is true — so it echoed a backtrace and threw Exception: test project ID, is mandatory, which nothing in the print flow catches → HTTP 500.

A valid test case always belongs to exactly one test project, so there was never a reason to render without resolving it first.

Considered alternatives:

1. Guard $info only (is_null() check around $info['issue_tracker_enabled']) — stops the crash but renders the test case without its project context: wrong prefix source, issue tracker interface silently skipped. Rejected as primary fix.
2. Derive the project id from the tree path in tcPrint.php and keep a defensive guard in the shared renderer. Chosen: the deep link gets the correct full context (prefix, custom fields, issue tracker), and any other caller that ever passes tproject_id=0 degrades gracefully instead of throwing.

Changes:

• lib/testcases/tcPrint.php: after validating the test case node, when $args->tproject_id <= 0 derive the owning project from tree::get_path($args->tcase_id). get_path() excludes the tree root, so element [0] is the child of the root and its parent_id is the test project id — the exact convention testcase::getPrefix() already uses.
• lib/functions/print.inc.php (initStaticRenderTestCaseForPrinting()): call get_by_id() only when $tprojectID > 0, and null-guard $info before reading issue_tracker_enabled.

• #553 — tlIssueTracker::getInterfaceObject() array-offset-on-null when issue_tracker_enabled=1 but no tracker linked.
• #554 — testproject::get_by_id() warns on positive-but-nonexistent ids ($result[0] on null).