java.lang.IllegalStateException: Not on FX application thread; currentThread = Thread-4
Solution 1:
Wrap timer.setText() in Platform.runLater(). Outside it, inside the while loop, add Thread.sleep(1000);
The reason behind Illegal State Exception is you are trying to update UI on some thread other than JavaFX Application thread.
The reason why your app was crashing when you added it was you were overloading the UI thread by adding a process to be executed on the UI thread infinitely. Making the thread sleep for 1000ms will help you overcome the issue.
If possible replace while(true) with a Timer or TimerTask.
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