fix an issue where switching charts back and forth can crash the app

Author: grobolom

src/main/java/de/dennisguse/opentracks/chart/ChartView.java

@@ -683,10 +683,18 @@ private record TitleDimensions(
      */
     private void drawSeriesTitles(Canvas canvas) {
         Iterator<TitlePosition> tpI = titleDimensions.titlePositions.iterator();
+
         for (ChartValueSeries chartValueSeries : seriesList) {
             if (chartValueSeries.isEnabled() && chartValueSeries.hasData() || allowIfEmpty(chartValueSeries)) {
                 String title = getContext().getString(chartValueSeries.getTitleId(unitSystem));
                 Paint paint = chartValueSeries.getTitlePaint();
+
+                // It is possible for the titlePositions to become empty temporarily, while switching between
+                // chart screens quickly.
+                if (!tpI.hasNext()) {
+                    return;
+                }
+
                 TitlePosition tp = tpI.next();
                 int y = topBorder - spacer - (titleDimensions.lineCount - tp.line) * (titleDimensions.lineHeight + spacer);
                 canvas.drawText(title, tp.xPos + getScrollX(), y, paint);