Try to load the project attached.
The FXML file contains references to resource files "Alert Dialog.*"
You will get the following exception :

Can't watch file: file:/Users/slions/tmp/template/Alert Dialog.png
java.lang.IllegalArgumentException: Illegal character in path at index 37: file:/Users/slions/tmp/template/Alert Dialog.png
at java.net.URI.create(URI.java:859)
at com.oracle.javafx.authoring.util.Utils.makeAbsoluteFile(Utils.java:1329)
at com.oracle.javafx.authoring.util.Utils.getAbsoluteFile(Utils.java:1349)
at com.oracle.javafx.authoring.util.Utils.toFile(Utils.java:1838)
at com.oracle.javafx.authoring.watch.ProjectWatcher.toFile(ProjectWatcher.java:812)
at com.oracle.javafx.authoring.watch.ProjectWatcher.retrievePropertyFiles(ProjectWatcher.java:387)
at com.oracle.javafx.authoring.watch.ProjectWatcher.visitElement(ProjectWatcher.java:631)
at com.oracle.javafx.authoring.watch.ProjectWatcher.init(ProjectWatcher.java:446)
at com.oracle.javafx.authoring.watch.ProjectWatcher.<init>(ProjectWatcher.java:434)
at com.oracle.javafx.authoring.Project.<init>(Project.java:566)
at com.oracle.javafx.authoring.Project.finish(Project.java:889)
at com.oracle.javafx.authoring.Project.forFxml(Project.java:805)
at com.oracle.javafx.authoring.Project.forFxml(Project.java:773)
at com.oracle.javafx.authoring.DesignerTool.loadFXMLLayout(DesignerTool.java:172)
at com.oracle.javafx.authoring.DesignerTool.loadFXMLLayout(DesignerTool.java:161)
at com.oracle.javafx.authoring.DesignerTool.openExistingFxml(DesignerTool.java:396)
at com.oracle.javafx.authoring.ApplicationMenu$8.handle(ApplicationMenu.java:439)
at com.oracle.javafx.authoring.ApplicationMenu$8.handle(ApplicationMenu.java:436)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:28)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.control.MenuItem.fire(MenuItem.java:456)
at com.sun.javafx.scene.control.GlobalMenuAdapter$6.handle(GlobalMenuAdapter.java:159)
at com.sun.javafx.scene.control.GlobalMenuAdapter$6.handle(GlobalMenuAdapter.java:157)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.control.MenuItem.fire(MenuItem.java:456)
at com.sun.javafx.tk.quantum.GlassSystemMenu$6.action(GlassSystemMenu.java:219)
Caused by: java.net.URISyntaxException: Illegal character in path at index 37: file:/Users/slions/tmp/template/Alert Dialog.png
at java.net.URI$Parser.fail(URI.java:2829)
at java.net.URI$Parser.checkChars(URI.java:3002)
at java.net.URI$Parser.parseHierarchical(URI.java:3086)
at java.net.URI$Parser.parse(URI.java:3034)
at java.net.URI.<init>(URI.java:595)
at java.net.URI.create(URI.java:857)
... 38 more
Can't watch file: file:/Users/slions/tmp/template/Alert Dialog.css
java.lang.IllegalArgumentException: Illegal character in path at index 37: file:/Users/slions/tmp/template/Alert Dialog.css
at java.net.URI.create(URI.java:859)
at com.oracle.javafx.authoring.util.Utils.makeAbsoluteFile(Utils.java:1329)
at com.oracle.javafx.authoring.util.Utils.getAbsoluteFile(Utils.java:1349)
at com.oracle.javafx.authoring.util.Utils.toFile(Utils.java:1838)
at com.oracle.javafx.authoring.watch.ProjectWatcher.toFile(ProjectWatcher.java:812)
at com.oracle.javafx.authoring.watch.ProjectWatcher.retrievePropertyFiles(ProjectWatcher.java:387)
at com.oracle.javafx.authoring.watch.ProjectWatcher.visitElement(ProjectWatcher.java:631)
at com.oracle.javafx.authoring.watch.ProjectWatcher.init(ProjectWatcher.java:446)
at com.oracle.javafx.authoring.watch.ProjectWatcher.<init>(ProjectWatcher.java:434)
at com.oracle.javafx.authoring.Project.<init>(Project.java:566)
at com.oracle.javafx.authoring.Project.finish(Project.java:889)
at com.oracle.javafx.authoring.Project.forFxml(Project.java:805)
at com.oracle.javafx.authoring.Project.forFxml(Project.java:773)
at com.oracle.javafx.authoring.DesignerTool.loadFXMLLayout(DesignerTool.java:172)
at com.oracle.javafx.authoring.DesignerTool.loadFXMLLayout(DesignerTool.java:161)
at com.oracle.javafx.authoring.DesignerTool.openExistingFxml(DesignerTool.java:396)
at com.oracle.javafx.authoring.ApplicationMenu$8.handle(ApplicationMenu.java:439)
at com.oracle.javafx.authoring.ApplicationMenu$8.handle(ApplicationMenu.java:436)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:28)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.control.MenuItem.fire(MenuItem.java:456)
at com.sun.javafx.scene.control.GlobalMenuAdapter$6.handle(GlobalMenuAdapter.java:159)
at com.sun.javafx.scene.control.GlobalMenuAdapter$6.handle(GlobalMenuAdapter.java:157)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.control.MenuItem.fire(MenuItem.java:456)
at com.sun.javafx.tk.quantum.GlassSystemMenu$6.action(GlassSystemMenu.java:219)
Caused by: java.net.URISyntaxException: Illegal character in path at index 37: file:/Users/slions/tmp/template/Alert Dialog.css
at java.net.URI$Parser.fail(URI.java:2829)
at java.net.URI$Parser.checkChars(URI.java:3002)
at java.net.URI$Parser.parseHierarchical(URI.java:3086)
at java.net.URI$Parser.parse(URI.java:3034)
at java.net.URI.<init>(URI.java:595)
at java.net.URI.create(URI.java:857)
... 38 more