To reproduce try to expand any column by dragging its header boundary with mouse.

You will get

java.lang.ClassCastException: javafx.scene.control.TreeTableColumn cannot be cast to javafx.scene.control.TableColumn
at javafx.scene.control.TableUtil.constrainedResize(TableUtil.java:253)
at javafx.scene.control.TreeTableView$5.call(TreeTableView.java:466)
at javafx.scene.control.TreeTableView$5.call(TreeTableView.java:455)
at javafx.scene.control.TreeTableView.resizeColumn(TreeTableView.java:1259)
...