timeline, sort: handle missing periodSpan in case of multiple periods

src/components/Timeline/TimelineChart.jsx

@@ -16,13 +16,14 @@ export const TimelineChart = (props) => {
     const xDomain = getTimeRangeOfTimelineData(props.filteredTimelineData,"period");
     const dataUnsorted = newGroupByPeriods(props.filteredTimelineData);
     const data = dataUnsorted && new Map([...dataUnsorted.entries()]
+        //sort by period start year
         .sort( (a,b) =>
-            a[1].periodSpan[0] - b[1].periodSpan[0] ));
+            a[1].periodSpan?.[0] - b[1].periodSpan?.[0] ));
 
     const timelineData = { xDomain, data, svgRef };
 
     //console.log("filteredTimelineData: ", props.filteredTimelineData);
-    //console.log("grouped by periods: ", data)
+    console.log("grouped by periods and sorted: ", data)
     //console.log("sorted data: ", dataUnsorted)
 
     //setting up the svg after first render