series.addRaceColumn() can now return null if it's impossible to add a race column

Added a null check

java/com.sap.sailing.server/src/com/sap/sailing/server/operationaltransformation/AddColumnToSeries.java

@@ -19,7 +19,7 @@ public class AddColumnToSeries extends AbstractColumnInSeriesOperation<RaceColum
         Series series = getSeries(toState);
         if (series != null) {
             result = series.addRaceColumn(getColumnName(), toState);
-            if (series.getRegatta().isPersistent()) {
+            if(result != null && series.getRegatta().isPersistent()) {
                 toState.updateStoredRegatta(series.getRegatta());
             }
         }