Item widgets: fix crash in mimeTypes()

*Always* check the result of a dynamic/qobject cast. I'm not positive
if the cast is correct (or should it just be a static cast),
but in principle these models can be used with a different view.

Change-Id: I37108cbb2744b6bf0fb46392d1cc9883f2d0b60e
Pick-to: 5.15
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>

src/widgets/itemviews/qlistwidget.cpp

@@ -463,7 +463,9 @@ void QListModel::itemChanged(QListWidgetItem *item, const QList<int> &roles)
 QStringList QListModel::mimeTypes() const
 {
     const QListWidget *view = qobject_cast<const QListWidget*>(QObject::parent());
-    return view->mimeTypes();
+    if (view)
+        return view->mimeTypes();
+    return {};
 }
 
 QMimeData *QListModel::internalMimeData()  const

src/widgets/itemviews/qtreewidget.cpp

@@ -738,7 +738,10 @@ QList<QTreeWidgetItem*>::iterator QTreeModel::sortedInsertionIterator(
 
 QStringList QTreeModel::mimeTypes() const
 {
-    return view()->mimeTypes();
+    auto v = view();
+    if (v)
+        return v->mimeTypes();
+    return {};
 }
 
 QMimeData *QTreeModel::internalMimeData()  const