angular tree component expand after loading data

I want to expand the first node on startup. I saw here the way to go. The difference is that I don't have the nodes in my code but get it from my databse. This is my Code:

this.appService.getTreeStructure().then((treeNodes) => {
      this.nodes = treeNodes;
      const treeModel: TreeModel = this.treeComponent.treeModel;
      console.log(treeModel);
      const firstNode: TreeNode = treeModel.getFirstRoot();
      console.log(firstNode);
      firstNode.expand();
    });

That doesnt work. the first console log shows that treeModel is an object but the second one is undefined. I can the a nodes property in treeModel that has allready all the nodes in it from the first output but I cant get access to them.

How can I expande the first node on startup?

When you update your nodes you need to listen to the updateData Event.

<tree-root [nodes]="nodes" (updateData)="onUpdateData()"></tree-root>

onUpdateData() {
      const treeModel: TreeModel = this.treeComponent.treeModel;
      const firstNode: TreeNode = treeModel.getFirstRoot();
      firstNode.expand();
}