Fixed crash when filtering class list

Issue: When you select an inner node (method or member) in classes
panel, then type something in search bar that filters that node out,
Kdevelop crashes. It seems that when nodes get filtered, at some
point currentIndex of QTreeView points to deleted node, which causes
segfault in ClassModel::hasChildren. Manually invalidating
currentIndex before filtering seems to have fixed the issue.

Differential Revision: https://phabricator.kde.org/D22660
BUG: 406931