Code updates

Added related issues that will help with the code added here.

After updating Charts to version 5.1.1., I also needed to update ChartsDrilldownPluginStyleChart.php of Charts Highcharts Drilldown (version 1.1.0), in line with this PHP error message:

PHP message: PHP Fatal error: Access level to Drupal\\charts_highcharts_drilldown\\Plugin\\views\\style\\ChartsDrilldownPluginStyleChart::getSelectedDataFields() must be protected

Changed:
line 235 in ChartsDrilldownPluginStyleChart.php
from
private function getSelectedDataFields(array $data_providers)
to
protected function getSelectedDataFields(array $data_providers): array
as per this commit suggested above for the 1.0.0-dev version.