@Override
    public CauseItem[] findCauses(TrackingRunner runner, PsiExpression expression, MemoryStateChange history) {
      FactDefinition<DfaNullability> nullability = history.findFact(history.myTopOfStack, FactExtractor.nullability());
      if (nullability.myFact == DfaNullability.NULLABLE || nullability.myFact == DfaNullability.NULL) {
        return new CauseItem[]{runner.findNullabilityCause(history, nullability.myFact)};
      }
      return new CauseItem[0];
    }
@Override
    public CauseItem[] findCauses(TrackingRunner runner, PsiExpression expression, MemoryStateChange history)