It's very easy to reproduce this:
jlquinn@cerberus:~/Pictures$ zenity --info --text='Hi there' Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
Ubuntu 16.04
It's very easy to reproduce this:
jlquinn@ cerberus: ~/Pictures$ zenity --info --text='Hi there'
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
Ubuntu 16.04