fix(test): correct test evaluation

- confirms Qt created a highlight before reset;
- treats a destroyed highlightItem as the valid hidden state after reset;
- no longer dereferences the stale highlight object.

Assisted-by: Codex:GPT-5
Signed-off-by: Rello <github@scherello.de>

Author: Rello

test/qml/activitieswindow/tst_activitieswindow.qml

@@ -64,7 +64,6 @@ Item {
         property ActivitiesWindow activitiesWindow
         property var activityList
         property var activityListView
-        property var activityHighlight
         property var newActivitiesButtonLoader
 
         function init()
@@ -134,8 +133,7 @@ Item {
         {
             activityListView.currentIndex = 0;
             tryVerify(() => activityListView.currentItem !== null);
-            activityHighlight = activityListView.highlightItem;
-            verify(activityHighlight);
+            tryVerify(() => activityListView.highlightItem !== null);
 
             activitiesWindow.requestActivate();
             activityListView.forceActiveFocus();
@@ -144,7 +142,7 @@ Item {
             activitiesWindow.resetActivityList();
 
             compare(activityListView.currentItem, null);
-            compare(activityHighlight.visible, false);
+            tryVerify(() => activityListView.highlightItem === null || !activityListView.highlightItem.visible);
         }
 
         function test_liveActivityDoesNotResetViewport()