title: Cannot read property push of null
body: <p>while pushing an  element for the first time to a child array which is null,I'm getting this error
  "Cannot read property push of null"
  But the element gets pushed,and the second time I do everything goes fine.It gets added to the array</p>
  
  <pre><code>this.group.departmentsList.push({
      name: group.newCategoryName,
      sortOrder: group.departmentsList.length,
      type: "category"
  });
  </code></pre>
  
  <p>group contains the data and departmentList is the child array which is declared like this:</p>
  
  <pre><code> $scope.parentDepartment = [
      {
          departmentsList: [{}]
      }
  ];
  </code></pre>