Unexpected error: Exception TypeError in RawEDF.__del__

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Hello all,

I am currently encountering the same error/warning:

Exception TypeError: TypeError('coercing to Unicode: need string or buffer,
NoneType found',) in <bound method RawEDF.__del__ of <RawEDF | s14.bdf,
n_channels x n_times : 69 x 9009152 (4399.0 sec), ~147 kB, data loaded>>
ignored

It does not occur consistently (i.e, I can't attribute to a particular
command), but I most often see it when I either reset variables (to clean
up the python environment) or when I delete a particular raw data object,
but not all the times. I have also seen it a couple of times after calling
plots (Raw, butterfly and PSD), just after I close the plot window.

I does not seem to affect anything, and as far as I can tell all calls and
commands run as expected, so I am not too concerned about it. The only
issue with it is that when I apply the same operations to a number of
individual datasets (in a loop), the mentioned message appears and
interrupts and exits the loop.

Has anyone encountered anything like this before? Is there something I
should consider or add into my code to avoid this? More importantly, am I
correct in assuming that this is a harmless warning, and ignoring it? Or is
there something I should fix?

Thanks for any comments / suggestions
Best,
Daniel

PS. I am using the latest version of MNE-python on a linux computer (Ubuntu
17.10), with Python 2.7. Please let me know if you need some more details
about my set-up.
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External Email - Use Caution

This is probably a harmless warning, yes. Are you passing a `str` to
preload (i.e., to use memmapping)? If so I think I see what might be
causing this. Feel free to open a GitHub issue and we can work to fix it.

Eric