I am not on the FX-ThreadThat's likely your issue. Anything which reads or writes the stuff displayed in the scene graph must be done on the FX application thread. Using those utilities which Richard mentions will make it so that the work is done on the FX application thread.
The reason I am "pretty sure" I am not "in" the FX-Thread is because I am using a SimpleDoubleProperty that I bind to. This is the same philosophy used to update a UI progress bar from within a worker thread.I wouldn't advise doing that. Binding anything which is UI related to anything which is updated in a worker thread will get you into thread troubles. The worker thread will update the property which will trigger the binding which will force an update of the bound UI control property (on the worker thread when the update to the UI control property should only have been triggered from the JavaFX application thread).