Gnuradio-3.7.1 : rx_ofdm.grc file does launches a barely visible window with no output

If I launch gr-digital/examples/ofdm/rx_ofdm.grc using
gnuradio-companion, it runs a barely visible window that is probably a
few 10x20 pixels in size, and if you maximize it, the window is empty
with no controls.

This is with the gnuradio-3.7.1 release.

Best regards,

Elvis D.