if the value property is null then you get an NPE from the renderer e.g.
java.lang.NullPointerException
at java.io.PrintWriter.write(PrintWriter.java:401)
at com.sun.faces.renderkit.html_basic.HtmlResponseWriter.write(HtmlResponseWriter.java:495)
at org.jboss.seam.ui.renderkit.FormattedTextRendererBase.doEncodeBegin(FormattedTextRendererBase.java:24)
at org.jboss.seam.ui.util.cdk.RendererBase.encodeBegin(RendererBase.java:79)
at javax.faces.component.UIComponentBase.encodeBegin(UIComponentBase.java:785)
at org.ajax4jsf.framework.renderer.RendererBase.renderChild(RendererBase.java:280)
at org.ajax4jsf.framework.renderer.RendererBase.renderChildren(RendererBase.java:262)
at org.richfaces.renderkit.TabRendererBase.encodeChildren(TabRendererBase.java:113)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:809)
at org.ajax4jsf.framework.renderer.RendererBase.renderChild(RendererBase.java:282)
at org.ajax4jsf.framework.renderer.RendererBase.renderChildren(RendererBase.java:262)
at org.richfaces.renderkit.html.TabPanelRenderer.doEncodeChildren(TabPanelRenderer.java:272)
at org.richfaces.renderkit.html.TabPanelRenderer.doEncodeChildren(TabPanelRenderer.java:267)
Cheers
J