wordpress-bootstrap-sidebar.php

<?php
array(
        'before_widget' => '<div class="panel panel-default panel-sidebar %1$s %2$s">',
        'after_widget' => '</div></div>',
        'before_title' => '<div class="panel-heading"><h3 class="panel-title">',
        'after_title' => '</h3></div><div class="panel-body">'
)
?>

<!-- If there is a title, it results in: -->

<div class="panel panel-default panel-sidebar">
<div class="panel-heading"><h3 class="panel-title">
Title!
</h3></div><div class="panel-body">
Contents!
</div></div>

<!-- …which is okay, but if there is no title, the layout falls into pieces: -->

<div class="panel panel-default panel-sidebar">
Contents!
</div></div>

<!-- because it should be this: -->

<div class="panel panel-default panel-sidebar">
<div class="panel-body">
Contents!
</div></div>