increaseRetryCount method Null safety

Implementation

void increaseRetryCount() {
  if (_backOffPeriod < config.maxInterval) {
    final newBackOff = config.interval * pow(config.multiplier, _retryCount);
    _backOffPeriod = min(newBackOff.toInt(), config.maxInterval.toInt());
  }

  _retryCount++;
}