I seem to remember a bug report or issue on that.
You might re-try in 5.7.1 – we've updated the code to hopefully fix that in that release.
It seems there is another issue with rendering page_selector widget at the block editing UI. When i use page_selector, the "Add" button is not working anymore. It may is some kind of interference between click events?
Try something like this at the \concrete\blocks\image_slider\form_setup_html.php inside 'imageTemplate' wrapper:
Also i wonder if it's possible at all to use page_selector at block editing ui offline? Or the only option would be fetching every item with ajax request? Maybe that's why the core image slider does not have page-selector widget for intrenal links?
Try something like this at the \concrete\blocks\image_slider\form_setup_html.php inside 'imageTemplate' wrapper:
<?php echo Loader::helper('form/page_selector')->selectPage('itemUrlInternal[]', (int)$itemUrl); ?>
Also i wonder if it's possible at all to use page_selector at block editing ui offline? Or the only option would be fetching every item with ajax request? Maybe that's why the core image slider does not have page-selector widget for intrenal links?