NPE with xxforms:itemset when control points to empty nodeset

<xhtml:html xmlns:xforms="http://www.w3.org/2002/xforms"
    xmlns:f="http://orbeon.org/oxf/xml/formatting"
    xmlns:xhtml="http://www.w3.org/1999/xhtml"
    xmlns:xxforms="http://orbeon.org/oxf/xml/xforms"
    xmlns:widget="http://orbeon.org/oxf/xml/widget"
    xmlns:ev="http://www.w3.org/2001/xml-events"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:fr="http://orbeon.org/oxf/xml/form-runner"
    xmlns:xbl="http://www.w3.org/ns/xbl"
    xmlns:xxbl="http://orbeon.org/oxf/xml/xbl"
    xmlns:exf="http://www.exforms.org/exf/1-0"
    xmlns:howto="http://www.orbeon.com/howto">

    <xhtml:head>
        <xhtml:title>Out of bound</xhtml:title>
        <xforms:model xxforms:xhtml-layout="span">

            <!-- Instance with state selected by user -->
            <xforms:instance>
                <instance>
                    <initial-state label="" valid=""/>
                    <removed-edit-state label="California" valid="">ca</removed-edit-state>
                    <removed-view-state label="California" valid="">ca</removed-view-state>
                </instance>
            </xforms:instance>

            <!-- Selected state is only valid if it is in the itemset -->
            <xforms:bind nodeset="*" constraint="@valid = 'true'"/>
            <xforms:bind nodeset="removed-view-state" readonly="true()"/>

            <!-- List of possible states -->
            <xforms:instance id="states">
                <states>
                    <state value="ma">Maryland</state>
                    <state value="ca">California</state>
                    <state value="co">Colorado</state>
                    <state value="nv">Nevada</state>
                </states>
            </xforms:instance>
        </xforms:model>

        <xbl:xbl>
            <xbl:binding id="howto-changing-itemset-select1" element="howto|changing-itemset-select1">
                <xbl:implementation>
                    <xforms:model>
                        <xforms:instance>
                            <alert/>
                        </xforms:instance>
                    </xforms:model>
                </xbl:implementation>
                <xbl:template>
                    <xforms:group appearance="xxforms:internal" id="container">
                        <!-- Pointers to nodes in the outside world -->
                        <xxforms:variable name="value"><xxforms:sequence xbl:attr="select=value-ref" xxbl:scope="outer"/></xxforms:variable>
                        <xxforms:variable name="label"><xxforms:sequence xbl:attr="select=label-ref" xxbl:scope="outer"/></xxforms:variable>
                        <xxforms:variable name="valid"><xxforms:sequence xbl:attr="select=valid-ref" xxbl:scope="outer"/></xxforms:variable>
                        <xxforms:variable name="alert" select="."/>
                        <xxforms:variable name="is-readonly" select="exf:readonly($value)"/>
                        <!-- Selection control -->
                        <xforms:select1 xbl:attr="appearance" ref="if (not($is-readonly)) then $value else ()" id="select1">
                            <xforms:dispatch ev:event="xforms-enabled xforms-value-changed" name="itemset-or-value-changed" target="container"/>
                            <xforms:alert ref="$alert"/>
                            <xforms:item>
                                <xforms:label/>
                                <xforms:value/>
                            </xforms:item>
                            <xbl:content/>
                        </xforms:select1>
                        <!-- Expose a "value" for the itemset, so we can have an event when it changes -->
                        <xforms:group ref="xxforms:itemset('select1', 'json')" id="itemset" appearance="xxforms:internal"/>
                        <!-- Update valid and alert when the value or the itemset changes  -->
                        <xxforms:variable name="is-out-of-bound" select="$value != '' and not($value = xxforms:itemset('select1', 'xml')//item/value)"/>
                        <xxforms:variable name="new-alert" select="if ($value = '') 
                            then 'A value is required'
                            else if ($is-out-of-bound)
                            then concat('A value was selected (', $label, ') and this value is not available anymore.')
                            else ''"/>
                        <xforms:action ev:event="xforms-enabled xforms-value-changed" ev:observer="select1 itemset">
                            <xxforms:variable name="new-label" select="xxforms:itemset('select1', 'xml')//item[value = $value]/label"/>
                            <xforms:setvalue if="exists($new-label)" ref="$label" value="$new-label"/>
                            <xforms:setvalue ref="$alert" value="$new-alert"/>
                            <xforms:setvalue ref="$valid" value="$alert = ''"/>
                        </xforms:action>
                        <xforms:output value="if ($is-out-of-bound) then $new-alert else ()"/>
                    </xforms:group>
                </xbl:template>
            </xbl:binding>
        </xbl:xbl>

        <xhtml:style type="text/css">
            p { margin-top: 2em }
            .previous-state { font-weight: bold }
            .xbl-fr-error-summary { margin-top: 1em; }
            .fr-error-summary-body { background-color: #F2D6C6; padding: .5em 0 .5em 0; width: 40em; }
            .xbl-fr-button { display: block; margin-top: 1em }
            .xbl-fr-error-summary .fr-error-alert { color: black; }
            .xbl-fr-error-summary .xforms-repeat-selected-item-1 { background-color: transparent }
            .xforms-alert-active { display: none }
            .xforms-invalid-visited .xforms-alert-active { display: inline-block }
        </xhtml:style>

    </xhtml:head>
    <xhtml:body>

        <xhtml:fieldset>
            <xhtml:legend>California is available</xhtml:legend>
            <xforms:group ref="initial-state">
                <howto:changing-itemset-select1 value-ref="." label-ref="@label" valid-ref="@valid" appearance="minimal" id="initial-state">
                    <xforms:label>State: </xforms:label>
                    <xforms:itemset nodeset="instance('states')/state">
                        <xforms:value ref="@value"/>
                        <xforms:label ref="."/>
                    </xforms:itemset>
                </howto:changing-itemset-select1>
                <fr:error-summary observer="initial-state"/>
            </xforms:group>
        </xhtml:fieldset>

        <xhtml:fieldset>
            <xhtml:legend>California selected but not available: edit case</xhtml:legend>
            <xforms:group ref="removed-edit-state">
                <howto:changing-itemset-select1 value-ref="." label-ref="@label" valid-ref="@valid" appearance="minimal" id="removed-edit-state">
                    <xforms:label>State: </xforms:label>
                    <xforms:itemset nodeset="instance('states')/state[@value != 'ca']">
                        <xforms:value ref="@value"/>
                        <xforms:label ref="."/>
                    </xforms:itemset>
                </howto:changing-itemset-select1>
                <fr:button>
                    <xforms:label>Save</xforms:label>
                    <xforms:dispatch ev:event="DOMActivate" name="fr-visit-all" targetid="removed-edit-state-error-summary"/>
                </fr:button>
                <fr:error-summary id="removed-edit-state-error-summary" observer="removed-edit-state"/>
            </xforms:group>
        </xhtml:fieldset>

        <xhtml:fieldset>
            <xhtml:legend>California selected but not available: view case</xhtml:legend>
            <xforms:group ref="removed-view-state">
                <howto:changing-itemset-select1 value-ref="." label-ref="@label" valid-ref="@valid" appearance="minimal" id="removed-view-state">
                    <xforms:label>State: </xforms:label>
                    <xforms:itemset nodeset="instance('states')/state[@value != 'ca']">
                        <xforms:value ref="@value"/>
                        <xforms:label ref="."/>
                    </xforms:itemset>
                </howto:changing-itemset-select1>
                <fr:error-summary id="removed-view-state-error-summary" observer="removed-view-state"/>
            </xforms:group>
        </xhtml:fieldset>

    </xhtml:body>
</xhtml:html>