QQuickItemViewTransitionAttached: Avoid dangling pointers

The items referred to here can be deleted when the animation finishes.
Use QPointer to zero them in that case.

Pick-to: 5.15 6.2
Fixes: QTBUG-84196
Change-Id: I695c9e91bd29d0583e4871d03ee946c40aa8a595
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/quick/items/qquickitemviewtransition.cpp

@@ -585,7 +585,7 @@ void QQuickItemViewTransitionableItem::stopTransition()
 
 
 QQuickViewTransitionAttached::QQuickViewTransitionAttached(QObject *parent)
-    : QObject(parent), m_item(nullptr), m_index(-1)
+    : QObject(parent), m_index(-1)
 {
 }
 /*!

src/quick/items/qquickitemviewtransition_p.h

@@ -226,7 +226,7 @@ private:
     QList<int> m_targetIndexes;
     QList<QObject *> m_targetItems;
 
-    QQuickItem *m_item;
+    QPointer<QQuickItem> m_item;
     int m_index;
 };
 

tests/auto/quickcontrols2/translation/data/stackView.qml

@@ -83,6 +83,12 @@ Window {
                 property int count: 0
 
                 onTriggered: {
+
+                    // This triggers the crash without retranslate().
+                    // retranslate() got cleverer in 6.2, sidestepping the issue.
+                    replaceExit.animations[0].target = null;
+                    replaceExit.animations[0].target = replaceExit.ViewTransition.item
+
                     console.log("timer within", root, "is about to call retranslate")
                     window.engine.retranslate()
                     window.calledTranslate()