Reproduced in build 060822.

To reproduce the bug:
- open the following schema
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            targetNamespace="http://xml.netbeans.org/schema/newXMLSchema"
            xmlns:tns="http://xml.netbeans.org/schema/newXMLSchema"
            elementFormDefault="qualified">
    <xsd:attributeGroup name="newAttributeGroup">
        <xsd:anyAttribute/>
    </xsd:attributeGroup>
    <xsd:complexType name="newComplexType">
        <xsd:attribute ref="tns:newAttribute"/>
        <xsd:attributeGroup ref="tns:newAttributeGroup"/>
    </xsd:complexType>
    <xsd:attribute name="newAttribute" type="xsd:string"/>
</xsd:schema>
- switch to Design;
- show attributes of newComplexType. It results in java.lang.ClassCastException:
org.netbeans.modules.xml.axi.impl.AnyAttributeProxy cannot be cast to
org.netbeans.modules.xml.axi.Attribute
	at
org.netbeans.modules.xml.schema.abe.StartTagPanel.chainIfNeeded(StartTagPanel.java:890)
	at
org.netbeans.modules.xml.schema.abe.StartTagPanel.chainAttributeGroups(StartTagPanel.java:857)
	at
org.netbeans.modules.xml.schema.abe.StartTagPanel.paintComponent(StartTagPanel.java:803)
	at
org.netbeans.modules.xml.schema.abe.ContentModelPanel$1.paintComponent(ContentModelPanel.java:65)