getVelocityEstimate method Null safety

Implementation

@override
VelocityEstimate getVelocityEstimate() {
  // The velocity estimated using this expression is an approximation of the
  // scroll velocity of an iOS scroll view at the moment the user touch was
  // released, not the final velocity of the iOS pan gesture recognizer
  // installed on the scroll view would report. Typically in an iOS scroll
  // view the velocity values are different between the two, because the
  // scroll view usually slows down when the touch is released.
  final Offset estimatedVelocity = _previousVelocityAt(-2) * 0.6
                                 + _previousVelocityAt(-1) * 0.35
                                 + _previousVelocityAt(0) * 0.05;

  final _PointAtTime? newestSample = _touchSamples[_index];
  _PointAtTime? oldestNonNullSample;

  for (int i = 1; i <= _sampleSize; i += 1) {
    oldestNonNullSample = _touchSamples[(_index + i) % _sampleSize];
    if (oldestNonNullSample != null) {
      break;
    }
  }

  if (oldestNonNullSample == null || newestSample == null) {
    assert(false, 'There must be at least 1 point in _touchSamples: $_touchSamples');
    return const VelocityEstimate(
      pixelsPerSecond: Offset.zero,
      confidence: 0.0,
      duration: Duration.zero,
      offset: Offset.zero,
    );
  } else {
    return VelocityEstimate(
      pixelsPerSecond: estimatedVelocity,
      confidence: 1.0,
      duration: newestSample.time - oldestNonNullSample.time,
      offset: newestSample.point - oldestNonNullSample.point,
    );
  }
}