Below is the HTML code I use for my button:
<button class="btn btn-outlined " ng-click="vm.showCommentBox1()">Notify All</button>
<div class="comment-box custom saveAll" ng-if=""><div class="panel panel-default">
<div class="panel-heading">
<button type="button" class="close" aria-label="Close" ng-click="comment=true" title="Close comment box"><span aria-hidden="true">×</span>
</button>
<h3 class="panel-title blue-30">Comment</h3>
</div>
<div class="panel-body">
<form>
<div class="form-group">
<textarea class="form-control bg-white" ng-model=""
placeholder="Add your description"></textarea>
</div>
</form>
</div>
</div>
</div>
By clicking on the 'x' button, the panel should close.