autodesk forge – TransformTool: undefined position

Im trying to implement the TransformTool extension and ran into some problems.
I can load it : .

But hovering over or moving the mouse throw this error : Transform.js:44 Uncaught TypeError: Cannot read properties of null (reading 'position').
From the call stack; the error originates from here :

onTxChange = () => {
    for (let fragId in this.selectedFragProxyMap) {
        let fragProxy = (this.selectedFragProxyMap)[fragId];
        console.log('pmmpOd ', fragProxy.position)

        fragProxy.position = new window.THREE.Vector3(
            this.transformMesh.position.x - fragProxy.offset.x,
            this.transformMesh.position.y - fragProxy.offset.y,
            this.transformMesh.position.z - fragProxy.offset.z);

        fragProxy.updateAnimTransform();

    }

    this.viewer.impl.sceneUpdated(true);
}

More precisely, this line fragProxy.position = new window.THREE.Vector3

fragProxy.position is defined before that line. I do know understand why it wont create it.

Thanks in advance for any help.