fixed NPE when validation is not available on revalidate call

src/main/java/ninja/javafx/smartcsv/fx/table/model/CSVModel.java

@@ -105,7 +105,7 @@ public class CSVModel {
      * walks through the data and validates each value
      */
     private void revalidate() {
-        if (header != null) {
+        if (header != null && validator != null) {
             validator.isHeaderValid(header);
         }