Set custom_virtualenv to null

Hello

I’v just updated AWX from 15.0.1 to 19.5.0.
I have some warning on AWX object where custom_virtualenv is still set to something.

I can’t found a way to set the setting custom_virtualenv to null.
It sounds this setting is in read only in the API.

Is there a way to remove this settings or set it to null in awx ?

Regards
Mickael