The exception :

Exception in thread "JavaFX Application Thread" org.eclipse.swt.SWTError: Cannot initialize Drop
at org.eclipse.swt.dnd.DND.error(Unknown Source)
at org.eclipse.swt.dnd.DND.error(Unknown Source)
at org.eclipse.swt.dnd.DropTarget.<init>(Unknown Source)
at javafx.embed.swt.FXCanvas$HostContainer.createDropTarget(FXCanvas.java:743)
at javafx.embed.swt.FXCanvas$HostContainer.setEmbeddedScene(FXCanvas.java:868)
at com.sun.javafx.tk.quantum.EmbeddedScene.setStage(EmbeddedScene.java:94)
at com.sun.javafx.tk.quantum.GlassStage.setScene(GlassStage.java:88)
at com.sun.javafx.tk.quantum.EmbeddedStage.setScene(EmbeddedStage.java:64)
at javafx.stage.Window$SceneModel.updatePeerScene(Window.java:491)
at javafx.stage.Window$SceneModel.invalidated(Window.java:460)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:111)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:145)
at javafx.stage.Window.setScene(Window.java:427)
at com.sun.javafx.stage.EmbeddedWindow.setScene(EmbeddedWindow.java:51)
at javafx.embed.swt.FXCanvas.setScene(FXCanvas.java:307)
at javaapplication34.FXCanvasApp.reset(FXCanvasApp.java:156)
at javaapplication34.FXCanvasApp$4.widgetSelected(FXCanvasApp.java:126)
at org.eclipse.swt.widgets.TypedListener.handleEvent(Unknown Source)
at org.eclipse.swt.widgets.EventTable.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Widget.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Unknown Source)
at org.eclipse.swt.widgets.Display.readAndDispatch(Unknown Source)
at javaapplication34.FXCanvasApp.main(FXCanvasApp.java:234)

Run the app, click button "Reset content"

As far as I remember, I didn't see such behavior before. Looks like recent issue