I get a similar error on Xfce with pamac-gtk3, but it works normally
$ pamac-manager
(pamac-manager:540049): Gdk-CRITICAL **: 12:49:25.131: gdk_window_set_cursor: assertion 'GDK_IS_WINDOW (window)' failed
If I redirect standard terminal messages (stdout) to /dev/null/ I still see this message
$ pamac-manager 1>/dev/null
(pamac-manager:543951): Gdk-CRITICAL **: 13:11:33.115: gdk_window_set_cursor: assertion 'GDK_IS_WINDOW (window)' failed
But if I redirect error message (stderr) message is not displayed
pamac-manager 2>/dev/null
IMO this is a non-critical error since pamac-gtk3 works normally