title: PHP Warning in Magento 2 "Warning: A non-numeric value encountered"
body: <p>I'm getting the following warning when trying to print order in Magento 2</p>
  <blockquote>
  <p>Warning: A non-numeric value encountered</p>
  </blockquote>
  <p>It points to line 336 of an extension I'm using. Line 336 contains</p>
  <pre><code> $this-&gt;lastPageFooterHeight = $fotterHeight + $elementGroupHeight;
  </code></pre>
  <p>As far as I'm aware, there shouldn't be any numerical values on this line. Some research suggests the issue could be with the + in the line. I've also read if could actually be down to the PHP version I'm using (7.3). Can anyone advise?</p>