Benign error from WMCore on checkwrite

This is what's printed at the very end of `crab checkwrite` [1]. This is the code/commit that's likely the cause [2]. Though the commit was intended to remove the error from the stdout, it somehow had the opposite effect and now appears in our stdout? Not sure how, seems mostly harmless at the moment, aside from possibly confusing the users.

[1]
```
Exception AttributeError: AttributeError("'NoneType' object has no attribute 'rmtree'",) in
> ignored
```
[2]
https://github.com/dmwm/WMCore/commit/01e8717c65685e472b417942965fe8f69d84e27b#diff-1671714d8798495de6896da37712c0a7L572