FB19400 during shutdown -- TypeError: Cannot read property 'buttons' of null

note: change also resolve a lot of other shutdown errors from QML (related to null parent)

interface/resources/qml/hifi/Desktop.qml

@@ -70,8 +70,8 @@ OriginalDesktop.Desktop {
         anchors.horizontalCenter: settings.constrainToolbarToCenterX ? desktop.horizontalCenter : undefined;
         // Literal 50 is overwritten by settings from previous session, and sysToolbar.x comes from settings when not constrained.
         x: sysToolbar.x
-        buttonModel: tablet.buttons;
-        shown: tablet.toolbarMode;
+        buttonModel: tablet ? tablet.buttons : null;
+        shown: tablet ? tablet.toolbarMode : false;
     }
 
     Settings {

interface/resources/qml/hifi/tablet/TabletHome.qml

@@ -115,9 +115,9 @@ Item {
             property int previousIndex: -1
             Repeater {
                 id: pageRepeater
-                model: Math.ceil(tabletProxy.buttons.rowCount() / TabletEnums.ButtonsOnPage)
+                model: tabletProxy != null ? Math.ceil(tabletProxy.buttons.rowCount() / TabletEnums.ButtonsOnPage) : null
                 onItemAdded: {
-                    item.proxyModel.sourceModel = tabletProxy.buttons;
+                    item.proxyModel.sourceModel = tabletProxy != null ? tabletProxy.buttons : null;
                     item.proxyModel.pageIndex = index;
                 }
 

libraries/qml/src/qml/OffscreenSurface.cpp

@@ -387,8 +387,16 @@ void OffscreenSurface::finishQmlLoad(QQmlComponent* qmlComponent,
         if (!parent) {
             parent = getRootItem();
         }
-        // Allow child windows to be destroyed from JS
-        QQmlEngine::setObjectOwnership(newObject, QQmlEngine::JavaScriptOwnership);
+        // manually control children items lifetime
+        QQmlEngine::setObjectOwnership(newObject, QQmlEngine::CppOwnership);
+        // some objects we are going to delete might be already deleted (children of parent we already deleted) so use QPointer to track lifetime
+        QPointer<QObject> trackedObject(newObject);
+        connect(_sharedObject, &SharedObject::onBeforeDestroyed, [trackedObject]() {
+            if (trackedObject) {
+                delete trackedObject.data();
+            }
+        });
+
         newObject->setParent(parent);
         newItem->setParentItem(parent);
     } else {

libraries/qml/src/qml/impl/SharedObject.cpp

@@ -79,9 +79,11 @@ SharedObject::SharedObject() {
 
 
 SharedObject::~SharedObject() {
+
+    emit onBeforeDestroyed();
+
     // After destroy returns, the rendering thread should be gone
     destroy();
-
     // _renderTimer is created with `this` as the parent, so need no explicit destruction
 #ifndef DISABLE_QML
     // Destroy the event hand

libraries/qml/src/qml/impl/SharedObject.h

@@ -67,6 +67,8 @@ public:
     bool isPaused() const;
     bool fetchTexture(TextureAndFence& textureAndFence);
 
+signals:
+    void onBeforeDestroyed();
 
 private:
     bool event(QEvent* e) override;