hidden box in post field

When having added a post simple field to posts, and trying to add a certain post, a box pops up which is completely empty. Using Chrome debug I found out thats its actually just the class hidden which causes that.

So somewhere in the plugin code there must be a bug that accidentally adds the hidden class to this div. Any idea where could be the bug?

Here some of the code from the added box:

<div class="simple-fields-meta-box-field-group-field-type-post-dialog hidden ui-dialog-content ui-widget-content" id="ui-id-1" style="display: block; width: auto; min-height: 61px; max-height: none; height: auto;">
					<p>Showing posts of type: <a href="tb_excercise">Excercises</a></p>

		<div class="simple-fields-meta-box-field-group-field-type-post-dialog-post-posts-wrap">
			<ul class="simple-fields-meta-box-field-group-field-type-post-dialog-post-posts">
				<li class=""><a href="wp-admin/post.php?post=4111&action=edit" data-post-id="4111">wrwr</a></li>			</ul>
		</div>
		<div class="submitbox">
			<div class="simple-fields-postdialog-link-cancel">
				<a href="#" class="submitdelete deletion">Cancel</a>
			</div>
		</div>
		</div>

https://wordpress.org/plugins/simple-fields/