title: try to update the text in a LABEL with OBSERVABLE but don't work
body: <p>I follow a lesson on nativescript and want to update the text in a LABEL (xml) with OBSERVABLE but my app crash.</p>
  
  <p>xml:</p>
  
  <pre><code>&lt;Page loaded = "mypageloaded"&gt;
      &lt;StackLayout&gt;
          &lt;Button text="TAP" tap="onButtonTap" /&gt;
          &lt;Label id = "lblcounter"  text="{{mylabel}}" /&gt;
      &lt;/StackLayout&gt;
  &lt;/Page&gt;
  </code></pre>
  
  <p>js:</p>
  
  <pre><code>var observableModule = require("tns-core-modules/data/observable");
  
  var counter = 42;
  var page;
  
  var user = new observableModule.fromObject({
    mylabel: "Test observable"
  });
  
  exports.mypageloaded = function (args) {
    page = args.objects;
    page.bindingContext = user;
  };
  
  exports.onButtonTap = function () {
    counter--;
    console.log(counter);
  }
  </code></pre>
  
  <p>if I put the line PAGE.BINDINGCONTEXT in comment, no crash...and no update...</p>