See JavaFX forum: https://forums.oracle.com/forums/thread.jspa?forumID=1385&threadID=2430102
This thread points to JFXtras as the source of the bug. But another user found otherwise:

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The getStyleClass().add("style"); call seems to be executed before the control is part of the scene in my control. This would lead to the warning because it could not find the styles from caspian.css. Because the layoutChildren() method will be called again after the control is part of the scene it will render like expected. So if I'm right I have to call the visualization code only if the control is part of a scene and the warnings should be gone right? I will check that later and let you know if that fixes the problem.
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The user confirms:

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just would like to let you know that the problem is fixed, it was really like I said in my last mail, now everything is fine and the warnings are gone.
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