Set default value for digits per group if loaded invalid value

src/main/java/name/nathanmcrae/numbersstation/MainController.java

@@ -134,6 +134,11 @@ public class MainController implements Initializable {
                 xmlMapper.writeValue(new File(filePath.toString()), settings);
             } else {
                 settings = xmlMapper.readValue(new File(filePath.toString()), MainSettings.class);
+                for (StationSettings station : settings.getStations()) {
+                    if (station.getDigitsPerGroup() == 0) {
+                        station.setDigitsPerGroup(4);
+                    }
+                }
             }
         } catch (IOException e) {
             // Print the contents of filePath