UndefinedError: 'None' has no attribute 'get_css_classes'
https://sentry.fsmpi.rwth-aachen.de/protokollsystem/protokollsystem/issues/13/
UndefinedError: 'None' has no attribute 'get_css_classes'
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File "program/server.py", line 693, in show_protocol
large_time_diff=large_time_diff, content_html=content_html)
UndefinedError: 'None' has no attribute 'get_css_classes'