fix(chart): Fixes an issue where the tooltip stayed open when the chart

was destroyed.

Fire the tooltipClose event when the chart is destroyed, so any open
tooltip will close when the parent chart component is destroyed.

Fixes dynatrace-oss#579

components/chart/src/chart.ts

@@ -459,6 +459,11 @@ export class DtChart
   }
 
   ngOnDestroy(): void {
+    // Next and complete the highChartsTooltipClosed observable
+    // to clear off any tooltips, that would still be open.
+    this._highChartsTooltipClosed$.next();
+    this._highChartsTooltipClosed$.complete();
+
     this._destroy$.next();
     this._destroy$.complete();
     if (this._chartObject) {