xfce4-panel don't show windows buttons with FlatSurfaces GTK3 theme

xfce4-panel with Flat Surfaces GTK3 theme don't show window buttons. How to reproduce:

1. Install theme https://github.com/cat-in-136/FlatSurfacesTheme
2. Close all windows (or restart the panel).
3. Open any new window (terminal, editor, other). Window buttons don't appear in task bar.

There are many errors in file .xsession-errors:

(xfce4-panel:6642): libtasklist-CRITICAL **: 22:43:22.758: tasklist-widget.c:1155 (xfce_tasklist_size_allocate): expression 'area.height == tasklist->size' failed.

(xfce4-panel:6642): libtasklist-CRITICAL **: 22:43:26.314: tasklist-widget.c:1155 (xfce_tasklist_size_allocate): expression 'area.height == tasklist->size' failed.

(xfce4-panel:6642): libtasklist-CRITICAL **: 22:43:26.341: tasklist-widget.c:1155 (xfce_tasklist_size_allocate): expression 'area.height == tasklist->size' failed.

• xfce4-panel, tested versions: 4.16.5, 4.17.3.
• GTK3: 3.24.34
• Distr: Debian (testing).

On screenshots: