Github user Mingun commented on the issue:
https://github.com/apache/jmeter/pull/394
Thanks!
I was tested with 2c82bf31db1110919e9ab84b13ab6c4f15d92ac5 (trunk branch).
The exception does not arise any more though to set *Undefined* value for File
property is impossible now, even though the flag {{NOT_UNDEFINED}} is not set
for property. I attach 2 screenshots showing behavior of the same plugin file
in versions 3.1 and 5.1:


Therefore this fix so far not really helps me
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