explode(): Argument #2 ($string) must be of type string, null given

Hi Georg, when i use this template (see docs):

<html xmlns:n="http://typo3.org/ns/GeorgRinger/News/ViewHelpers"
      xmlns:f="http://typo3.org/ns/TYPO3/CMS/Fluid/ViewHelpers" data-namespace-typo3-fluid="true">

<f:layout name="General" />
<!--
    =====================
        Templates/Category/List.html
-->

<f:section name="content">
    <f:if condition="{categories}">
        <f:then>
            <f:render section="categoryTree" arguments="{categories:categories,overwriteDemand:overwriteDemand}" />
        </f:then>
        <f:else>
            <f:translate key="list_nocategoriesfound" />
        </f:else>
    </f:if>
</f:section>

<f:section name="categoryTree">
    <ul>
        <f:for each="{categories}" as="category">
            <f:variable name="categoryUid">{category.item.uid}</f:variable>
            <f:if condition="{category.item.sysLanguageUid} > 0">
                <f:variable name="categoryUid">{category.item.l10nParent}</f:variable>
            </f:if>
            <li>
                <n:multiCategoryLink.isCategoryActive list="{overwriteDemand.categories}" item="{categoryUid}">
                    <f:then>
                        <f:link.page title="{category.item.title}" class="active" pageUid="{settings.listPid}"
                            additionalParams="{tx_news_pi1:{overwriteDemand:{categories: categoryUid}}}">{category.item.title}
                        </f:link.page>

                        (<f:link.page title="{category.item.title}" class="active" pageUid="{settings.listPid}"
                                      additionalParams="{n:multiCategoryLink.arguments(mode:'remove',item:category.item.uid,list:overwriteDemand.categories)}">remove

                    </f:link.page>)

                    </f:then>
                    <f:else>
                        <f:link.page title="{category.item.title}" pageUid="{settings.listPid}"
                            additionalParams="{tx_news_pi1:{overwriteDemand:{categories: categoryUid}}}">{category.item.title}
                        </f:link.page>

                        (<f:link.page title="{category.item.title}" class="active" pageUid="{settings.listPid}"
                                     additionalParams="{n:multiCategoryLink.arguments(mode:'add',item:category.item.uid,list:overwriteDemand.categories)}">add
                        </f:link.page>)

                    </f:else>
                </n:multiCategoryLink.isCategoryActive>

                <f:if condition="{category.children}">
                    <f:render section="categoryTree" arguments="{categories: category.children,overwriteDemand:overwriteDemand}" />
                </f:if>
            </li>
        </f:for>
    </ul>
</f:section>
</html>

it appears an error like:

But when i delete the list:overwriteDemand.categories in the add button it works only in single category mode!

georgringer / news_filter

explode(): Argument #2 ($string) must be of type string, null given #22