I'm using Arcgis api for javascript 4.5, and got following issue after mouse over on the map a few times:

Here's my mouse over handler:
this.config.view.on("pointer-move", (event) => this._pointerMoveHandler(event));
_pointerMoveHandler(event: any) {
// the hitTest() checks to see if any graphics in the view
// intersect the given screen x, y coordinates
this.config.view.hitTest(event)
.then((event) => this._showGraphicTooltip(event))
.otherwise((event) => this._closeToolTipDialog());
}
Here's MapView source code:

Could anyone please help? It's an urgent issue.
Much appreciated!