Hotfix for recent arsnova.cards migration process
W20210119-13:15:33.990(1)? (STDERR) TypeError: Cannot read property 'leitner' of undefined
W20210119-13:15:33.990(1)? (STDERR) at Function.setCardTimelineStats (imports/util/leitner.js:863:19)
W20210119-13:15:33.990(1)? (STDERR) TypeError: Cannot read property 'leitner' of undefined
W20210119-13:15:33.990(1)? (STDERR) at Function.setCardTimelineStats (imports/util/leitner.js:863:19)