Wayland: incorrect About dialog appears for different plugins

This applies to plugins that have About option in the right click context menu. Clicking About opens about dialog for the selected plugin, but only for the first time. Clicking About on another plugin shows info of the firstly selected one. Pressing About for any other plugin will always show the About dialog of the firstly selected plugin. Not an issue on x11.

Steps to reproduce:

1. Use wayland (tested with xfwl4 and labwc)
2. Have various plugins in the panel (for example PulseAudio plugin and Power Manager plugin)
3. On the panel, right click on PulseAudio plugin, select About
4. About PulseAudio dialog appears, close it
5. Right click on Power Manager plugin, select About
6. About PulseAudio dialog appears, but About Power Manager dialog should appear
7. Select any other plugin, click about, about PulseAudio will always appear