[Bug 1893021]

[Stransky]

I think you can safely ignore that, that comes from Gtk3 library which
complains about error in actual Gtk theme. Eventually you can change
your Gtk theme in gnome-tweaks application.

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https://bugs.launchpad.net/bugs/1893021

Title:
  Flash plugin symbol lookup error with firefox 80.0+build2 on xenial
  (gtk_window_set_titlebar)

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