Error: The QName value 'topic' does not resolve to a(n) element declaration

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
    >

    <xs:element name='xmap-content'>
        <xs:complexType>
            <xs:sequence>
                <xs:element name='sheet'>
                    <xs:complexType>
                        <xs:sequence>

                            <xs:element name='topic'>
                                <xs:complexType>
                                    <xs:sequence minOccurs="0" maxOccurs="unbounded">
                                        <xs:element name='title' type='xs:string'/>
                                        <xs:element name='children' minOccurs="0" maxOccurs="1">
                                            <xs:complexType>
                                                <xs:sequence>
                                                    <xs:element name='topics'>
                                                        <xs:complexType>
                                                            <xs:sequence>
                                                                <xs:element ref='topic' minOccurs="0" maxOccurs="unbounded"/>
                                                            </xs:sequence>
                                                        </xs:complexType>
                                                    </xs:element>
                                                </xs:sequence>

                                            </xs:complexType>
                                        </xs:element>
                                    </xs:sequence>
                                </xs:complexType>
                            </xs:element>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
</xs:schema>