When instantiating timelines, this is usually done with a series of key frames. However, the creation of each key frame requires the instantiation of both a KeyFrame, a Duration, and a KeyValue object, like this
new Timeline(
    new KeyFrame(new Duration(100), new KeyValue(sendingLabel.opacityProperty(), 1.0)),
    new KeyFrame(new Duration(800), new KeyValue(sendingLabel.opacityProperty(), 0.0))
);
I recommend this is simplified by introducing 1 additional constructor in KeyFrame, which will allow me to write the code above as follows:
new Timeline(
    new KeyFrame(100, sendingLabel.opacityProperty(), 1.0),
    new KeyFrame(800, sendingLabel.opacityProperty(), 0.0)
);
I hope it is evident to the reader, that this involves much less typing and is easier to learn, easier to read, and less expensive to maintain.