Fix/workaround GTK libtree issue with multi-unit symbols

When preparing for clearing the tree, GTK requires walking through
parents.  After calling "Freeze()" to protect against returning bad data
(see #6458 and #4471), we can no longer access Parent() from GTK and the
tree cannot be cleared.

The fix is to collapse the first element if it is open.  This prevents
the common case of the history elements being expanded in the tree

Fixes https://gitlab.com/kicad/code/kicad/issues/6102

common/lib_tree_model_adapter.cpp

@@ -139,17 +139,25 @@ void LIB_TREE_MODEL_ADAPTER::DoAddLibrary( wxString const& aNodeName, wxString c
 void LIB_TREE_MODEL_ADAPTER::UpdateSearchString( wxString const& aSearch )
 {
     {
-        // DO NOT REMOVE THE FREEZE/THAW. This freeze/thaw is a flag for this model adapter
-        // that tells it when it shouldn't trust any of the data in the model. When set, it will
-        // not return invalid data to the UI, since this invalid data can cause crashes.
-        // This is different than the update locker, which locks the UI aspects only.
         wxWindowUpdateLocker updateLock( m_widget );
 
+        // This collapse is required before the call to "Freeze()" below.  Once Freeze()
+        // is called, GetParent() will return nullptr.  While this works for some calls, it
+        // segfaults when we have our first library expanded.
+        // The tree will be expanded again below when we get our matches
+        if( !aSearch.IsNull() )
+            m_widget->Collapse( wxDataViewItem( &*m_tree.Children[0] ) );
+
         // Even with the updateLock, wxWidgets sometimes ties its knickers in
         // a knot when trying to run a wxdataview_selection_changed_callback()
         // on a row that has been deleted.
         // https://bugs.launchpad.net/kicad/+bug/1756255
         m_widget->UnselectAll();
+
+        // DO NOT REMOVE THE FREEZE/THAW. This freeze/thaw is a flag for this model adapter
+        // that tells it when it shouldn't trust any of the data in the model. When set, it will
+        // not return invalid data to the UI, since this invalid data can cause crashes.
+        // This is different than the update locker, which locks the UI aspects only.
         Freeze();
         BeforeReset();