Error while running event toolbars-mod::on_gui_location_changed (ID 158)
...sections/section/content/table/slots/item/ItemButton.lua:25: attempt to index field 'elem_tooltip' (a nil value)
stack traceback:
...sections/section/content/table/slots/item/ItemButton.lua:25: in function 'new'
toolbars-mod/src/gui/Component.lua:149: in function 'loadChildren'
toolbars-mod/src/gui/Component.lua:132: in function 'load'
toolbars-mod/src/player/Player.lua:73: in function 'load'
toolbars-mod/src/player/Player.lua:48: in function 'get'
toolbars-mod/control.lua:57: in function <toolbars-mod/control.lua:54>

I guess it was because I added and removed several other mods.

What version of the factorio engine and mod?
You were loading a game with already existing toolbars?
Can you try with fresh game, create new toolbars, save game and load it again?

What version of the factorio engine and mod?

Last from experimental branch

You were loading a game with already existing toolbars?

Yes

Can you try with fresh game, create new toolbars, save game and load it again?

I've disabled "Toolbars", load game, save game, enable "toolbars" again, load game - all is fine, just I lost my configured toolbars :)

BTW, it would be a c00l feature, if we can save/load configuration of toolbars separately from the savegame :)

It's in Todo. New version of factorio is introducing fix regarding tooltips which I reported. It could removed tooltips completely for some reason even if it shouldn't.

Toolbars shouldn't disappear event after turning off the mod.

I've just started 2.0.13 stable and it works fine. Maybe you had some specific item in the toolbar which caused failure.