Click "submit" directs to wp-admin/admin-ajax.php

When I click on Submit, instead of cycling back to the same page, I end up in an empty page (except for a 0 at the top left), at the address wp-admin/admin-ajax.php.

$sc =  '[fu-upload-form class="your-class" title="1) Upload your Travel photographs" action="?" success_page="?"]';
$sc .= '[input type="file" name="photo" id="ug_photo" class="required" description="" multiple="true"]';
$sc .= '[input type="text" name="destinationField" id="destinationField" class="required" description="<h2 class=destination>2) Type your Destination here:</h2>" ]';
$sc .= '[input type="submit" class="btn" id="submission" value="3) Let MyTravelLog build Your Blog!"]';
$sc .= '[/fu-upload-form]';
echo "<div id='Editor_1'>";
echo do_shortcode($sc);

https://ww.wp.xz.cn/plugins/frontend-uploader/