GWT Binder error problem? show [ERROR] Element may only contain a single child element, but found and . Element

[samuel]

I got the problem when I compile my project. I write my error .xml
file.

<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
        xmlns:g='urn:import:com.google.gwt.user.client.ui'>
        <g:DockLayoutPanel ui:field="dock">
                <g:north size="25">
                        <g:FlowPanel>
                                <g:Button ui:field="save">Save</
g:Button>
                        </g:FlowPanel>
                </g:north>
                <g:center>
                        <g:VerticalPanel ui:field="panel1">
                                <g:Button ui:field="button1">print</
g:Button>
                                <g:Button ui:field="button2">clear</
g:Button>
                        </g:VerticalPanel>
                  <g:VerticalPanel ui:field="panel2">
                                <g:TextArea ui:field="textArea" />
                        </g:VerticalPanel>
                </g:center>
        </g:DockLayoutPanel>
</ui:UiBinder>

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