strip_tags(): Passing null to parameter #1 ($string) of type string is deprecated in template_preprocess_footable_view()

Patch created

-    elseif (drupal_strlen($field_handler->last_render) != drupal_strlen(strip_tags($field_handler->last_render))) {
+    elseif (drupal_strlen($field_handler->last_render) != drupal_strlen(strip_tags((string) $field_handler->last_render ?? ''))) {

If $field_handler->last_render is not a string, it is pointless to execute that code, as zero (drupal_strlen($field_handler->last_render)) is equal to zero (drupal_strlen(strip_tags((string) $field_handler->last_render ?? '')).

Addressed the comment #4, So to address this issue modified the code to check if $field_handler->last_render is a string before executing the drupal_strlen and strip_tags functions.
Before attempting to check here its length and strip out any tags, If $field_handler->last_render is not a string, the code inside the if statement will not be executed.

So updating the patch and attaching Interdiff.