Handle invalid model indices gracefully

data() being called with an invalid model index seems to happen during
normal operations here, with something in the proxy model chain not
catching that case (due to a row being filtered out presumably?).

src/models/messageeventmodel.cpp

@@ -434,7 +434,9 @@ static LinkPreviewer *emptyLinkPreview = new LinkPreviewer;
 
 QVariant MessageEventModel::data(const QModelIndex &idx, int role) const
 {
-    Q_ASSERT(checkIndex(idx, QAbstractItemModel::CheckIndexOption::IndexIsValid));
+    if (!checkIndex(idx, QAbstractItemModel::CheckIndexOption::IndexIsValid)) {
+        return {};
+    }
     const auto row = idx.row();
 
     if (!m_currentRoom || row < 0 || row >= int(m_currentRoom->pendingEvents().size()) + m_currentRoom->timelineSize()) {