"Not on FX application thread; currentThread = JavaFX Application Thread" and "InvocationTargetException" - error-handling

Question

I'm beginner in java so need help.
I have a javafx app that works with Apache POI (for Excel files).
When I click menubutton I have an Error in Controller class which should load Excel data in Map collection and then show new Stage window.
But I have a strange error "Not on FX application thread; currentThread = JavaFX Application Thread" when press the button.
THere is a list of red masseges in output window with such main titles:
Exception in thread "JavaFX Application Thread"
java.lang.RuntimeException:
java.lang.reflect.InvocationTargetException
Caused by: java.lang.reflect.InvocationTargetException
Caused by: java.lang.IllegalStateException: Not on FX application thread;
currentThread = JavaFX Application Thread
Here is the code for Method in Controller
public void SKULoading(ActionEvent ae) {
Stage stage = (Stage) menuBar.getScene().getWindow();
stage.hide();
XSSFWorkbook wb = ExcelModel.OpenFile(stage); //using Filechooser
ExcelModel book = new ExcelModel(wb);
Map<Integer, List<String>> data = book.SheetToArray(0);
book.WBclose();
Stage primarystage;
// Since that moment error occurs!
primarystage = new Stage();
FXMLLoader loader = new FXMLLoader();
Pane root = null;
try {
root = loader.load(getClass().getResource("/FProject/SKUmanager.fxml").openStream());
} catch (IOException ex) {
Logger.getLogger(MainWindowController.class.getName()).log(Level.SEVERE, null, ex);
}
Scene scene = new Scene(root);
primarystage.setScene(scene);
primarystage.show();
}
I saw this topic How to avoid Not on FX application thread; currentThread = JavaFX Application Thread error? but solution like "Platform.runLater(new Runnable(){});" seems to solve the problem if I wrap in it almost all code in the Method. but I'm not sure it is right desicion because I didn't create any new Thread before.
When I removed row stage.hide(); new Stage was shown
Or When I left row stage.hide(); and removed rows
XSSFWorkbook wb = ExcelModel.OpenFile(stage);
ExcelModel book = new ExcelModel(wb);
Map<Integer, List<String>> data = book.SheetToArray(0);
book.WBclose();
Stage was shown again.
When I replace code
FileChooser fileChooser = new FileChooser();
fileChooser.setTitle("Open Resource File");
File file = fileChooser.showOpenDialog(stage);
in method ExcelModel.OpenFile(stage); by this
File file = new File("C:\\Users\\Maks\\Desktop\\Test.xlsx");
Stage was also shown.
So, I think, answer is lying near creating/hiding Stages or using FileChooser (Looks like using Filechooser somehow influences on the appearence of a new Stage after privious Stage was hidden)
UPD
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1405)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$343(ContextMenuContent.java:1358)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:352)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:275)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:388)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:387)
at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
at com.sun.glass.ui.View.notifyMouse(View.java:937)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
... 43 more
Caused by: java.lang.IllegalStateException: Not on FX application thread; currentThread = JavaFX Application Thread
at com.sun.javafx.tk.Toolkit.checkFxUserThread(Toolkit.java:236)
at com.sun.javafx.tk.quantum.QuantumToolkit.checkFxUserThread(QuantumToolkit.java:423)
at javafx.stage.Stage.<init>(Stage.java:241)
at javafx.stage.Stage.<init>(Stage.java:227)
at FProject.MainWindowController.SKULoading(MainWindowController.java:124)
... 53 more
Help me to find out what is the root of problem to avoid such errors in future. Thanks!