"Wrong type argument" error while setting "pyvenv-activate" variable with "elpy-set-project-variable"

[SergiyKolesnikov]

I am trying to set pyvenv-activate as a project variable using elpy-set-project-variable function:
M-x elpy-set-project-variable RET pyvenv-activate RET

And a this point I get the following error: widget-file-prompt-value: Wrong type argument: stringp, nil

If there is a .dir-locals.el file with a value for pyvenv-activate already, e.g.:

;;; Directory Local Variables
;;; For more information see (info "(emacs) Directory Variables")

((nil . ((pyvenv-activate . ""))))

then setting the value using elpy-set-project-variable works fine.

pyvenv 20201227.1623

Elpy Configuration
Emacs.............: 27.1
Elpy..............: 1.35.0
Virtualenv........: None
Interactive Python: ipython3 7.20.0 (/usr/bin/ipython3)
RPC virtualenv....: rpc-venv (/home/user/.emacs.d/elpy/rpc-venv)
Python...........: python3 3.9.2 (/home/user/.emacs.d/elpy/rpc-venv/bin/python3)
Jedi.............: 0.17.2 (0.18.0 available)
Rope.............: 0.18.0
Autopep8.........: 1.5.4 (1.5.6 available)
Yapf.............: 0.30.0 (0.31.0 available)
Black............: 20.8b1
Syntax checker....: flake8 (/home/user/.emacs.d/elpy/rpc-venv/bin/flake8)