Fix 'Warning: Trying to access array offset on value of type null in...

Fix 'Warning: Trying to access array offset on value of type null in Drupal\cmrf_form_processor\FormProcessorBaseHandler->webformSubmissionToApiParams()'

Closes #3476730

src/FormProcessorBaseHandler.php

@@ -285,7 +285,7 @@ abstract class FormProcessorBaseHandler {
           }
         }
       } else {
-        if ($fields[$key]['format'] == 'do_not_submit') {
+        if (isset($fields[$key]['format']) && $fields[$key]['format'] == 'do_not_submit') {
           continue;
         }
         $element = $this->webformHandler->getWebform()->getElement($key);