Messagebox.show always return value 1, irrespective of the selection button

public void onClick$btn_delete{
    if (Messagebox.OK == Messagebox.show("Are you sure to delete?", "Question", Messagebox.OK | Messagebox.CANCEL, Messagebox.QUESTION)))  {
           ... ...
    }
}

public List<?> getList(){
 try{ 
       ... ...
  }catch(Exception e){
    Messagebox.show("... ...");
  }
}

<zk>
	<zscript><![CDATA[
		public void show(){
			try{ 
			    throw new Exception();
			}catch(Exception e){
				if (Messagebox.OK == 
					Messagebox.show("Are you sure to delete?", "Question", 
							Messagebox.OK | Messagebox.CANCEL, Messagebox.QUESTION))  {
				      System.out.println("in");
				}
			}
		}
	]]></zscript>
	<button label="click" onClick="show();"/>
</zk>

<?xml version="1.0" encoding="UTF-8"?>
<zk>

	<!-- Turn on the following if you want to use the trendy mold as default
		for all buttons.
	-->
	<library-property>
		<name>org.zkoss.zul.Button.mold</name>
		<value>trendy</value>
	</library-property>
	<library-property>
		<name>org.zkoss.zul.Button.autodisable</name>
		<value>true</value>
	</library-property>
	<!-- [ZK EE]
		Turn on if you want to enable the render-on-demand feature for grid.
		It improves the performance for huge grid.
		Default: false
	-->
	<library-property>
		<name>org.zkoss.zul.grid.rod</name>
		<value>true</value>
	</library-property>
	<!-- [ZK EE]
		Turn on if you want to enable the render-on-demand feature for listbox.
		It improves the performance for huge listbox.
		Default: false
	-->
	<library-property>
		<name>org.zkoss.zul.listbox.rod</name>
		<value>true</value>
	</library-property>
	
	<library-property>
		<name>org.zkoss.zul.progressbox.position</name>
		<value>center,top</value>
	</library-property>
	
	<system-config>
	<disable-event-thread>false</disable-event-thread>
	</system-config>
	
	<session-config>
		<session-timeout>1800</session-timeout>
	</session-config>
	
	<session-config>
		<device-type>ajax</device-type>
		<timeout-uri>/timeout.zul</timeout-uri>
	</session-config>

	<listener>
		<description> Mointor the statistic</description>
		<listener-class>org.zkoss.zk.ui.util.Statistic</listener-class>
	</listener>
</zk>

<?xml version="1.0" encoding="UTF-8"?>
<?variable-resolver class="org.zkoss.zkplus.spring.DelegatingVariableResolver"?>
<?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit" arg0="./bw_dialog"?>
<zk xmlns="http://www.zkoss.org/2005/zul"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.zkoss.org/2005/zul http://www.zkoss.org/2005/zul/zul.xsd">
	<window id="bw_dialog" apply="${bwCtrl}">
		<listbox id="listbox_bw" checkmark="true" multiple="true"
			model="@{bwCtrl.bw_list}" mold="paging" pageSize="10">
			<listhead>
				<listheader label="name"/>
			</listhead>
			<listitem self="@{each=one}">
				<listcell label="@{one}" />
			</listitem>
		</listbox>
	</window>
</zk>

@Controller("bwCtrl")
@Scope("prototype")
public class BlackWhiteDialogCtrl extends GenericForwardComposer {

	private Textbox tbox_name;

	public void doAfterCompose(Component comp) throws Exception {
		super.doAfterCompose(comp);
                Messagebox.show("Hello world."); // the exception happens
	}

	public List<String> getBw_list() {
		List<String> list = null;
		try {
			throw new Exception();
		} catch (Exception e) {
			try {
				if (Messagebox.OK == 
					Messagebox.show("Are you sure to delete?", "Question", 
							Messagebox.OK | Messagebox.CANCEL, Messagebox.QUESTION))  {
					Messagebox.show("OK.");
				}else{
					Messagebox.show("wrong.");
				}
			} catch (InterruptedException e1) {
				e1.printStackTrace();
			}
		}
		return list;
	}

}

Servlet.service() for servlet  in context with path [] threw exception
org.zkoss.zk.ui.SuspendNotAllowedException: doModal must be called in an event listener
	at org.zkoss.zul.Window.doModal(Window.java:535)
	at org.zkoss.zul.Messagebox.show(Messagebox.java:244)
	at org.zkoss.zul.Messagebox.show(Messagebox.java:266)
	at com.pcmp.control.zk.BWCtrl.doAfterCompose(BWCtrl.java:24)

public class BlackWhiteDialogCtrl  extends GenericForwardComposer {
		
			public void onCreate(){
				Messagebox.show("Hello world."); // the exception happens
			}
		}

	public void onCreate() {
		try {
			if (null == getBw_list()) {
				Messagebox.show("getBw_list() error!");
				return;
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public List<String> getBw_list() throws Exception {
		List<String> list = null;
		try {
			throw new Exception();
		} catch (Exception e) {
			e.printStackTrace();
		}
		return list;
	}

try {
	Messagebox.show("Question is pressed. Are you sure?", "Question",
		Messagebox.OK | Messagebox.CANCEL, Messagebox.QUESTION, new EventListener() {
			@Override
			public void onEvent(Event event) throws Exception {
				if (((Integer) event.getData()).intValue() != Messagebox.OK) {
					System.out.println("Messagebox.CANCEL selected!");
					return;
				} else {
					System.out.println("Messagebox.OK selected!");
					return;
				}		
		});
} catch (InterruptedException e) {
	e.printStackTrace();
}

public void onClick$deleteButton() {
     try {
	     Messagebox.show("Question is pressed. Are you sure?", "Question",
	          Messagebox.OK | Messagebox.CANCEL, Messagebox.QUESTION, new EventListener() {
			@Override
			public void onEvent(Event event) throws Exception {
				if (((Integer) event.getData()).intValue() != Messagebox.OK) {
					System.out.println("Messagebox.CANCEL selected!");
					return;
				} else {
					//Executes the delete logic
					return;
				}		
		});
     } catch (InterruptedException e) {
	e.printStackTrace();
     }
}

<grid model="@{win$composer.dataList, load-after='deleteButton.onClick'}">