Segfault in prefs dialog for some launcher config

Crash reported in libxfce4windowing#35 (comment 98980) with a video, where a backtrace leads to https://gitlab.xfce.org/xfce/xfce4-panel/-/blob/09fc3106b2ec5db3f3d5b730e1bb4b8383699d36/panel/panel-preferences-dialog.c#L1282