gnuradio 3.9 on ubuntu 20.04

Hi all,

I upgrade the system to ubuntu 20.04 starting from iso.
Then I install GNURADIO 3.9.0.0 following the instruction in the wiki.

GNU radio is correctly installed because I'm able to run successfully top_block.py but I have problems building the flowgraf.
In attached picture of the error that I get : waveforms generator don't show frequency (even do in block editor is correctly assigned).

Of course I'm not able to generate any topblock.

I try to re-install gnuradio but get same error.

On internet I found some indication that the problem can be in grc.conf file global_block_patch but not working on my cases.

Thank you for help.

David