Fixed: every load of lib/general/mainPage.php that switched/initialized the session Test Project while the session still held a stale Test Plan (a plan belonging to the previous test project) wrote three E_WARNING entries into the Event Viewer whenever the newly selected project had no accessible test plans.

Event Viewer gained three entries per switch (log_level=2):

E_WARNING Undefined array key 0            - lib/general/mainPage.php - Line 102
E_WARNING Trying to access array offset on null - lib/general/mainPage.php - Line 102
E_WARNING Undefined array key 0            - lib/general/mainPage.php - Line 105

1. Fresh DB; create test project "Alpha" with one active test plan, test project "Beta" with none. Log in as admin.
2. Select Alpha (session testplanID = Plan A).
3. Request lib/general/mainPage.php?tproject_id=<Beta> — any project switch via the nav bar does exactly this.
4. Event Viewer → the three warnings above.

The block in mainPage.php that re-validates the session test plan ran only when $testplanID > 0, but never checked that $arrPlans was non-empty:

$arrPlans = (array)$currentUser->getAccessibleTestPlans($db,$testprojectID);
if($testplanID > 0) {
    ...
    if( $found == 0 ) {
        $index = 0;
        $testplanID = $arrPlans[$index]['id'];   // line 102: empty array!
    }
    setSessionTestPlan($arrPlans[$index]);       // line 105: empty array!
    $arrPlans[$index]['selected']=1;
}

When switching projects, initProject() validates the old plan id against the new project (getAccessibleTestPlans($db,$tproject,$tplan)), gets null (the old plan is not in it), falls back to "any accessible plan of the new project", also gets null (Beta has none) — and leaves the stale session testplanID untouched. Back in mainPage.php the stale id is > 0, so the block runs with $arrPlans = []:

• $arrPlans[0] → Undefined array key 0, evaluating to null,
• null['id'] → Trying to access array offset on null,
• second $arrPlans[0] at line 105 → Undefined array key 0.

PHP 8.x logs all three as E_WARNING into the Event Viewer.

Considered alternatives:

1. Null-coalesce / isset() around each read (the issue's minimal suggestion). Silences the warnings but leaves the stale plan id flowing into $gui->testplanID while $countPlans == 0 — an inconsistent state (page claims a selected plan that belongs to another project).
2. Guard + explicitly drop the stale session plan (chosen): when the fallback list is empty there is nothing selectable, so reset the local $testplanID to 0 and call setSessionTestPlan(null) — its documented else-branch unsets $_SESSION['testplanID'] / $_SESSION['testplanName'], which is exactly what the buggy code was accidentally achieving anyway (it fed null into setSessionTestPlan(), just with warnings).

if( $found == 0 ) {
    // update test plan id
    if(count($arrPlans) > 0) {
      $index = 0;
      $testplanID = $arrPlans[$index]['id'];
    }
    else {
      // Session still points at a Test Plan of ANOTHER Test Project while
      // the newly selected one has no accessible Test Plans. Nothing can be
      // selected here: drop the stale session plan instead of reading
      // offset 0 of an empty array (PHP 8.x turns this into E_WARNINGs,
      // see GitHub issue #548). setSessionTestPlan(null) clears the keys.
      $testplanID = 0;
      setSessionTestPlan(null);
    }
}

if($testplanID > 0) {
    setSessionTestPlan($arrPlans[$index]);
    $arrPlans[$index]['selected']=1;
}

Behavior parity for the healthy paths is preserved exactly: found-match keeps the matched index; found-miss with a non-empty list still falls back to index 0; no plan in session skips the whole block as before.

• lib/general/mainPage.php — guarded the found == 0 branch and made the session-write/selected-marking conditional on a resolved plan.