Pentaho Spoon displays many errors like this in console:
(SWT:12688): GLib-GObject-WARNING **: /build/glib2.0-7IO_Yw/glib2.0-2.48.1/./gobject/gclosure.c:724: unable to remove uninstalled invalidation notifier: 0x7f1f2a537bc0 (0x7f1f6d5af530)
The only adverse effect of these errors seems to be somewhat slower performance.
Ubuntu 16.04, Gnome, Pentaho 6.1 Community Edition.
After some research, it helped to install this library:
sudo apt install libswt-gnome-gtk-4-jni