click item in list -> view details but no output? Firebase.child failed: First argument was an invalid path

sorry for another post, still learning. Please help with the problem. Also would be happy if someone could explain their answers too. Thanks!

The items show up in a list format with the html code below.

<div class="post row" ng-repeat="(postId, post) in posts">
   <a href="{{ post.url }}">{{ post.title }}</a>

But when I put the same code without the ng-repeat into the details page, it has no output.

Below are the outputs for the 2 differennt scenarios:

1)If i use ng-click="view(postId)", with a more complicated ui-state, with view defined;

$scope.view = function(postId) {
      $state.go('tab.posts.view', {postId: postId});
    };, 

it displays ()

2) If I use ui-sref="tab.view({postId:post.id})", with a simpler ui-router state;

it displays {{post.title}}

But both are bad output!

Update: F12 error

Firebase.child failed: First argument was an invalid path: "". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]"