Ribbon Tab Behaviour

Realised that when I use the following CustomAction,

 <?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <CustomAction Id="Chapter3.CustomTab" Location="CommandUI.Ribbon" RegistrationType="List" RegistrationId ="101">
    <CommandUIExtension>
      <CommandUIDefinitions>
        <CommandUIDefinition Location="Ribbon.Tabs._children">
          <Tab Id="Chapter3.CustomTab" Title="Chapter3" Description="Demo tab for Chapter 3" Sequence="501">
            <Scaling Id="Chapter3.CustomTab.Scaling">
              <MaxSize Id="Chapter3.FirstDemoGroup.MaxSize"
                       GroupId="Chapter3.FirstDemoGroup"
                       Size="OneLarge"/>
              <Scale Id="Chapter3.FirstDemoGroup.Scaling.CustomTabScaling"
                     GroupId="Chapter3.FirstDemoGroup"
                     Size="OneMedium" />
            </Scaling>
            <Groups Id="Chapter3.Groups">
              <Group Id="Chapter3.FirstDemoGroup"
                     Description="Contains demo controls"
                     Title="Demo Group"
                     Sequence="52"
                     Template="Ribbon.Templates.SingleButton">
                <Controls Id="Chapter3.FirstDemoGroup.Controls">
                  <Button Id="Chapter3.FirstDemoGroup.HelloWorld"
                          Command="Chapter3.HelloWorldCommand"
                          Sequence="15"
                          Image16by16="/_layouts/images/NoteBoard_16x16.png"
                          Image32by32="/_layouts/images/NoteBoard_32x32.png"
                          Description="Display a hello world msg"
                          LabelText="Hi World"
                          TemplateAlias="c1"
                          />
                </Controls>
              </Group>
            </Groups>
          </Tab>
        </CommandUIDefinition>
        <CommandUIDefinition Location="Ribbon.Templates._children">
          <GroupTemplate Id="Ribbon.Templates.SingleButton">
            <Layout Title="OneLarge" LayoutTitle="OneLarge">
              <Section Alignment="Top" Type="OneRow">
                <Row>
                  <ControlRef DisplayMode="Large" TemplateAlias="c1"/>
                </Row>
              </Section>
            </Layout>
            <Layout Title="OneMedium" LayoutTitle="OneMedium">
              <Section Alignment="Top" Type="OneRow">
                <Row>
                  <ControlRef DisplayMode="Medium" TemplateAlias="c1"/>
                </Row>
              </Section>
            </Layout>
          </GroupTemplate>
        </CommandUIDefinition>
      </CommandUIDefinitions>
      <CommandUIHandlers>
        <CommandUIHandler Command="Chapter3.HelloWorldCommand"
           CommandAction="javascript:
            var notificationId = SP.UI.Notify.addNotification('Hello World');" />
      </CommandUIHandlers>
    </CommandUIExtension>
  </CustomAction>
</Elements>

And if you remove the CommandUIHandlers component, the tab will not appear!!!!!!!! Strange...