The dock stopped working after updating Ubuntu Mate 19.10 to 20.04

I upgraded Ubuntu Mate 19.10 to 20.04 and now I can't use the dock and get the following crash report:

 Traceback (most recent call last):
   File "/usr/lib/mate-applets/mate-dock-applet/dock_applet.py", line 57, in <module>
     from gi.repository import MatePanelApplet
   File "/usr/lib/python3/dist-packages/gi/importer.py", line 140, in load_module
     raise ImportError(e)
 ImportError: Requiring namespace 'Gtk' version '3.0', but '2.0' is already loaded

I reported the issue here:

but it seems that the issue is generated by some package that remains from Ubuntu Mate 19.10.

I reset the panel layout and even download and install the last version.