JavaFX loading FXML causes warning and fails to load the file (skin/modena)

Question

Situation

I'm trying to build a JavaFX App and for GUI I use SceneBuilder to produce fxml files for the different roots to use. Then I launch the App and at some point I'm trying to load some fxml file with new FXMLLoader(App.class.getResource("myFXML.fxml")).load() and set it as root on the scene on the stage.

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Issue

99% of the times the loading has no problem at all and everything works as intended, but that 1% will fail and prevent the fxml loading without stopping the App but only throws the warning bellow.

WARNING: Caught 'java.lang.ClassCastException: class java.lang.String 
cannot be cast to class javafx.scene.paint.Paint (java.lang.String is in
module java.base of loader 'bootstrap'; javafx.scene.paint.Paint is in 
module javafx.graphics@18 of loader 'app')' while converting value for 
'-fx-background-color' from rule '*.list-view' in stylesheet 
jar:file:///C:/mysystem/.m2/repository/org/openjfx/javafx-controls/18/javafx-controls-18-win.jar!/com/sun/javafx/scene/control/skin/modena/modena.bss

The fxml that causes it most of the time is the one with a ComboBox in it but i don't think this has to do with it since it happened anywhere.

Important the warning is not consistent as well, at the pre-last line instead of *.list-view other times says *.button as well, but i am doing the same action to test it.

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Reproducing the Issue

Reproducing is not consistent at all, the warning is descriptive and seems that the problem is on a JavaFX file (mentioned in the warning) and not in something I wrote. Thus I can't find a consistent way to reproduce it or handle the exception or something.

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Tried so far

• Removing the ComboBox (just in case)
• Searched the internet to find nothing about it
• Re-installing JDK did nothing

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Note

I know there is much more information I can provide that may help but I'm exhausted right now. Maybe will add with edit later on.

Answer 1

Long Story Short

I was doing the FXML loading ( new FXMLLoader(App.class.getResource("myFXML.fxml")).load() ) and scene.setRoot(...) on another Thread and not the FX Thread. So make sure with Platform.isFxApplicationThread(); (as mentioned in the comments) that the operation happens on the FX Thread to ensure expected behavior.

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Important

The answer is not complete , it is a placeholder for anyone searching it currently and this resolves the issue.

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Improvements

I will try as soon as possible to update the question with more information and reproducible example. Also I will with the help of the comments continue the research to find solid explanation for the issue. In the mean time feel free for edits or post new better answers.