User deprecated function "Setting slot value to an array is not supported" when message is output

Problem

When requesting a custom elements page in I just got this error:

"error": [
    "<em class=\"placeholder\">User deprecated function</em>: Setting slot value to an array is not supported. Provide a CustomElement, MarkupInterface or a markup string. Treating content as a render array. in <em class=\"placeholder\">Drupal\\custom_elements\\CustomElement-&gt;setSlot()</em> (line <em class=\"placeholder\">300</em> of <em class=\"placeholder\">modules/contrib/custom_elements/src/CustomElement.php</em>). <pre class=\"backtrace\">Drupal\\custom_elements\\CustomElement-&gt;setSlot(&#039;field_image&#039;, Array, &#039;div&#039;, Array) (Line: 29)\nDrupal\\custom_elements\\Processor\\FileReferenceFieldItemListProcessor-&gt;addtoElement(Object, Object, &#039;full&#039;) (Line: 138)\nDrupal\\custom_elements\\CustomElementGenerator-&gt;process(Object, Object, &#039;full&#039;) (Line: 100)\nDrupal\\custom_elements\\Processor\\DefaultContentEntityProcessor-&gt;addtoElement(Object, Object, &#039;full&#039;) (Line: 138)\nDrupal\\custom_elements\\CustomElementGenerator-&gt;process(Object, Object, &#039;full&#039;) (Line: 119)\nDrupal\\custom_elements\\CustomElementGenerator-&gt;generate(Object, &#039;full&#039;) (Line: 48)\nDrupal\\lupus_ce_renderer\\Controller\\CustomElementsController-&gt;entityView()\ncall_user_func(Array) (Line: 53)\nDrupal\\lupus_ce_renderer\\EventSubscriber\\CustomElementsControllerSubscriber-&gt;Drupal\\lupus_ce_renderer\\EventSubscriber\\{closure}() (Line: 158)\nSymfony\\Component\\HttpKernel\\HttpKernel-&gt;handleRaw(Object, 1) (Line: 80)\nSymfony\\Component\\HttpKernel\\HttpKernel-&gt;handle(Object, 1, 1) (Line: 57)\nDrupal\\Core\\StackMiddleware\\Session-&gt;handle(Object, 1, 1) (Line: 47)\nDrupal\\Core\\StackMiddleware\\KernelPreHandle-&gt;handle(Object, 1, 1) (Line: 106)\nDrupal\\page_cache\\StackMiddleware\\PageCache-&gt;pass(Object, 1, 1) (Line: 85)\nDrupal\\page_cache\\StackMiddleware\\PageCache-&gt;handle(Object, 1, 1) (Line: 47)\nDrupal\\Core\\StackMiddleware\\ReverseProxyMiddleware-&gt;handle(Object, 1, 1) (Line: 52)\nDrupal\\Core\\StackMiddleware\\NegotiationMiddleware-&gt;handle(Object, 1, 1) (Line: 23)\nStack\\StackedHttpKernel-&gt;handle(Object, 1, 1) (Line: 706)\nDrupal\\Core\\DrupalKernel-&gt;handle(Object) (Line: 19)\n</pre>"

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes