How to Retrieve Forge Viewer objectTree?

Question

My goal is to highlight a room by adding new geometry to the viewer based on lines I have created in revit like they do here Link

but i can not figure out how to access

Answer 1

Shiya Luo was correct the viewer had not yet finished loading the geometry

in my extentions Load function I added two event listeners and made sure they both fired before trying to access the instanceTree

 viewer.addEventListener(Autodesk.Viewing.GEOMETRY_LOADED_EVENT, function () {
    finishedGEOMETRY_LOADED_EVENT = true;
    if(finishedGEOMETRY_LOADED_EVENT && finishedOBJECT_TREE_CREATED_EVENT ){
        afterModelLoadEvents(viewer);
    }
 });
viewer.addEventListener(Autodesk.Viewing.OBJECT_TREE_CREATED_EVENT, function () {
    finishedOBJECT_TREE_CREATED_EVENT = true;
    if(finishedGEOMETRY_LOADED_EVENT && finishedOBJECT_TREE_CREATED_EVENT ){
        afterModelLoadEvents(viewer);           
    }
 });