occasional (ignored) exception AttributeError: 'NoneType' object has no attribute 'deleteDataHandle'

[KrisThielemans]

We sometimes (no idea when exactly) get

Exception ignored in: <function ImageData.__del__ at 0x7bc1b8ceee80>
Traceback (most recent call last):
File "/opt/SIRF-SuperBuild/INSTALL/python/sirf/STIR.py", line 447, in __del__
AttributeError: 'NoneType' object has no attribute 'deleteDataHandle'

It seems harmless, but is worrying, and potentially causes problems in PETRIC2.

[KrisThielemans]

Suggested steps:

• do we have any of this when running the tests? Run ctest -VVV
• if we do, which ones?
• for those, check with valgrind or similar (likely means RelWithDebInfo build)