Magento – Magento 2.2 error after upgrade

Seems that this is happening because of the block class path, where the 'category' folder is written with lower case. Try to change it to 'Category'.

I would advise using Require JS to load this rather than adding it via XML like this, but if you're adamant you want to do it this way try this:

<!-- Bugherd -->
<referenceBlock name="head.additional">
    <block class="Magento\Framework\View\Element\Text" name="my_script">
        <action method="setText">
            <argument translate="true" name="text" xsi:type="string">
                <![CDATA[
                    <script type='text/javascript'>
                    (function (d, t) {
                      var bh = d.createElement(t), s = d.getElementsByTagName(t)[0];
                      bh.type = 'text/javascript';
                      bh.src = 'https://www.bugherd.com/sidebarv2.js?apikey=dkjfsd89id6sftsdghkjn3';
                      s.parentNode.insertBefore(bh, s);
                      })(document, 'script');
                    </script>
                ]]>
             </argument>
        </action>
     </block>
</referenceBlock>

Same as mtr.web's answer but uses class rather than block.