(Unity:23929): Gtk-CRITICAL **: 14:06:35.951: gtk_widget_get_scale_factor: assertion 'GTK_IS_WIDGET (widget)' failed

Editor Version 2018.3.11,Can’t load scene. Editor Version 2018.3.11
OS: Ubuntu 18.04

(Unity:23929): Gtk-CRITICAL **: 14:06:35.951: gtk_widget_get_scale_factor: assertion ‘GTK_IS_WIDGET (widget)’ failed

Try this:

  1. In your project, delete the Library folder. I found that CurrentLayout-default.dwlt in particular must be deleted, so you can try that first if your project is big and want to avoid long rebuilding times.
  2. In ~/.config/unity3d/Preferences/Editor-5.x, delete the Layouts folder
  3. In ~/.config/unity3d, delete the Browser folder
  4. In ~/.config/unity3d, delete the Editor folder (there is one sub-folder per editor version but the serial number cannot be trivially matched with a human-readable version, so maybe just delete the whole folder)
  5. Restart the project

I found 1, 2 & 3 to work in my case, and only 1+2 or 2+3 not to work. At least it should work when installing a new editor / re-installing an editor version.
I also did 4 at some point so it may be needed for editor versions already installed.
I did 2 at the beginning so not sure if required.