Ubuntu Dock extension error

After installing the Dash To Dock extension, the default dock disappeared, but it still reappears sometimes in the Activities view.

Also, the default Ubuntu Dock extension is reported with an exclamation mark in the Tweaks Extensions panel, and it says: "Error loading extension". If I disable Dash To Dock, the default dock comes back normally, but it still reports the same error in the Tweaks panel.

Any ideas what's happening?

Thanks.