Add missing t_str() call to fix wxUSE_STL build after r73792.

Don't rely on implicit conversion from wxString to wxChar* which is not
available in all builds.

Closes #15145.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73808 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Check that focused window is non-NULL before using it in wxComboCtrl.

wxWindow::FindFocus() may return NULL, don't crash by attempting to
dereference the pointer returned by it.

Closes #15146.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73809 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775