Fix NPE

- Backport #593
- Fixes #598

spring-shell-core/src/main/java/org/springframework/shell/component/flow/BaseSingleItemSelector.java

@@ -118,7 +118,7 @@ public abstract class BaseSingleItemSelector extends BaseInput<SingleItemSelecto
 	@Override
 	public SingleItemSelectorSpec preHandler(Consumer<SingleItemSelectorContext<String, SelectorItem<String>>> handler) {
 		this.preHandlers.add(handler);
-		return null;
+		return this;
 	}
 
 	@Override