TableExport Error ?? (NullPointerException)

Hi
TableExport Error ....
help me.

maven dependency

        <dependency>
           <groupId>org.vaadin.addon</groupId>
           <artifactId>tableexport-for-vaadin</artifactId>
           <version>1.5.1.5</version>
        </dependency>
        <repository>
            <id>vaadin-addons</id>
            <url>http://maven.vaadin.com/vaadin-addons</url>
        </repository>

final ThemeResource export = new ThemeResource("../../../excel2.bmp");
final Button excelExportButton = new Button("Export");
excelExportButton.setIcon(export);
excelExportButton.addClickListener(new Button.ClickListener() {
    private ExcelExport excelExport;
    private Table table = new Table("excel");
    @Override
    public void buttonClick(ClickEvent event) {
        table.addContainerProperty("Name", String.class, null);
        table.addContainerProperty("Mag",  Float.class, null);

        // Add a row the hard way
        Object newItemId = table.addItem();
        Item row1 = table.getItem(newItemId);
        row1.getItemProperty("Name").setValue("Sirius");
        row1.getItemProperty("Mag").setValue(-1.46f);

        // Add a few other rows using shorthand addItem()
        table.addItem(new Object{"Canopus",        -0.72f}, 2);
        table.addItem(new Object{"Arcturus",       -0.04f}, 3);
        table.addItem(new Object{"Alpha Centauri", -0.01f}, 4);
        
        excelExport = new ExcelExport(table);
        excelExport.excludeCollapsedColumns();
        excelExport.setReportTitle("Excel Report");
        excelExport.setDisplayTotals(false);
        excelExport.export();
    }
});
        
        
Jboss7 Error Message
09:50:02,766     
[com.vaadin.server.DefaultErrorHandler] (http-localhost-127.0.0.1-8080-7) : java.lang.NullPointerException
    at com.vaadin.addon.tableexport.TableExport.sendConvertedFileToUser(TableExport.java:92) [tableexport-for-vaadin-1.5.1.5.jar:1.5.1.5]
    at com.vaadin.addon.tableexport.ExcelExport.sendConverted(ExcelExport.java:378) [tableexport-for-vaadin-1.5.1.5.jar:1.5.1.5]
    at com.vaadin.addon.tableexport.TableExport.export(TableExport.java:68) [tableexport-for-vaadin-1.5.1.5.jar:1.5.1.5]
    at kr.co.skinfosec.admin.view.normalization.NormalizationPolicyTableEditor$4.buttonClick(NormalizationPolicyTableEditor.java:240) [classes:]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.7.0_79]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) [rt.jar:1.7.0_79]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) [rt.jar:1.7.0_79]
    at java.lang.reflect.Method.invoke(Unknown Source) [rt.jar:1.7.0_79]
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:508) [vaadin-server-7.5.2.jar:7.5.2]
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:198) [vaadin-server-7.5.2.jar:7.5.2]
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:161) [vaadin-server-7.5.2.jar:7.5.2]
    at com.vaadin.server.AbstractClientConnector.fireEvent(AbstractClientConnector.java:1003) [vaadin-server-7.5.2.jar:7.5.2]
    at com.vaadin.ui.Button.fireClick(Button.java:393) [vaadin-server-7.5.2.jar:7.5.2]
    at com.vaadin.ui.Button$1.click(Button.java:61) [vaadin-server-7.5.2.jar:7.5.2]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.7.0_79]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) [rt.jar:1.7.0_79]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) [rt.jar:1.7.0_79]
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