A DESCRIPTION OF THE PROBLEM :
The javadoc for the method invalidated(...) in the javafx.beans.InvalidationListener states:

This method needs to be provided by an implementation of InvalidationListener. It is called if an Observable becomes invalid.
In general is is considered bad practice to modify the observed value in this method.
                 ^
There should be an "it" in there, rather than an "is".

https://docs.oracle.com/javase/10/docs/api/javafx/beans/InvalidationListener.html