ng-if check if array is empty
post.capabilities.items will still be defined because it's an empty array, if you check post.capabilities.items.length it should work fine because 0 is falsy.
Verify the length property of the array to be greater than 0:
<p ng-if="post.capabilities.items.length > 0"> <strong>Topics</strong>: <span ng-repeat="topic in post.capabilities.items"> {{topic.name}} </span></p>Arrays (objects) in JavaScript are truthy values, so your initial verification <p ng-if="post.capabilities.items"> evaluates always to true, even if the array is empty.
To overcome the null / undefined issue, try using the ? operator to check existence:
<p ng-if="post?.capabilities?.items?.length > 0">Sidenote, if anyone got to this page looking for an Ionic Framework answer, ensure you use
*ngIf:<p *ngIf="post?.capabilities?.items?.length > 0">