Fix race: Object (m_item) is operated on before it is assigned.

The race happens in:

QObject *obj = component->create(ctx);
m_item = qobject_cast<QQuickItem *>(obj);

Truncated crash:

#0  0x00007ffff5546354 in QQmlContext::parentContext() const () at /usr/lib/libQt5Qml.so.5
#1  0x00007fff68362cf3 in DelegateRecycler::syncModelProperties() (this=0x55555d3a9210) at delegaterecycler.cpp:162
#2  0x00007fff68336367 in DelegateRecycler::qt_static_metacall() at moc_delegaterecycler.cpp:229
#3  0x00007fff683365bd in DelegateRecycler::qt_metacall() at moc_delegaterecycler.cpp:308

src/delegaterecycler.cpp

@@ -127,7 +127,7 @@ DelegateRecycler::~DelegateRecycler()
 void DelegateRecycler::syncIndex()
 {
     const QVariant newIndex = m_propertiesTracker->property("trackedIndex");
-    if (!newIndex.isValid()) {
+    if (!m_item || !newIndex.isValid()) {
         return;
     }
     QQmlContext *ctx = QQmlEngine::contextForObject(m_item)->parentContext();
@@ -137,7 +137,7 @@ void DelegateRecycler::syncIndex()
 void DelegateRecycler::syncModel()
 {
     const QVariant newModel = m_propertiesTracker->property("trackedModel");
-    if (!newModel.isValid()) {
+    if (!m_item || !newModel.isValid()) {
         return;
     }
     QQmlContext *ctx = QQmlEngine::contextForObject(m_item)->parentContext();
@@ -156,7 +156,7 @@ void DelegateRecycler::syncModel()
 void DelegateRecycler::syncModelProperties()
 {
     const QVariant model = m_propertiesTracker->property("trackedModel");
-    if (!model.isValid()) {
+    if (!m_item || !model.isValid()) {
         return;
     }
     QQmlContext *ctx = QQmlEngine::contextForObject(m_item)->parentContext();
@@ -174,7 +174,7 @@ void DelegateRecycler::syncModelProperties()
 void DelegateRecycler::syncModelData()
 {
     const QVariant newModelData = m_propertiesTracker->property("trackedModelData");
-    if (!newModelData.isValid()) {
+    if (!m_item || !newModelData.isValid()) {
         return;
     }
     QQmlContext *ctx = QQmlEngine::contextForObject(m_item)->parentContext();