Here is the extension which has a warning:
<extension
point="org.jboss.tools.common.text.xml.contentAssistProcessor"
id="org.jboss.tools.seam.ui.contentAssistProcessor"
name="org.jboss.tools.seam.ui.contentAssistProcessor">
</extension>
It says that this extensions requires at least one contentAssistProcessor element.
Please fix the xsd of this extension point if plugin.xml is correct.