title: How to save mat tree expansion model and apply it?
body: <p>I need to save current expanded nodes of mat tree close all and then expand them all again.
  My approach was like this:</p>
  <p>Save the expansion model</p>
  <pre><code>treeControl = new NestedTreeControl&lt;NavbarNode&gt;((node: NavbarNode) =&gt; return node.children);
  
   this.prevExpansionModel = this.treeControl.expansionModel.selected;
        this.treeControl.collapseAll();
  </code></pre>
  <p>and then apply it</p>
  <pre><code>this.prevExpansionModel.forEach(object =&gt; this.treeControl.expand(object));
  </code></pre>
  <p>It works when I do it this way but the nodes that got expanded will no longer work and I can't toggle them. (I cannot toggle those that have been expanded, other ones I can expand and work normally)</p>