Fix assert when editing an item in multi-selection wxTreeCtrl.

Fix assert when editing an item in multi-selection wxTreeCtrl.

Don't use GetSelection() in wxTreeCtrl EVT_CHAR_HOOK handler as it can only be
used with single selection controls and results in an assert when used with
multi-selection ones. Simply use GetFocusedItem() instead.

Closes #14062.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70795 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775