Use is to compare values with None

[temp20230620]

isinstance throws an exception when used with None.

Thanks for merging the previous ones!

[mschuett]

Thank you for creating the PRs :)

I learned now that isinstance(x, NoneType) would work too. But PEP8 recommends "Comparisons to singletons like None should always be done with is or is not", so this change is the right one.