Monai Label for QuPath not compatible with QuPath 0.5.x

Here is the error log when instaling the extension into QuPath 0.5.1

Error loading extension qupath.lib.extension.monailabel.Extension@46956954: qupath/lib/gui/ActionTools
java.lang.NoClassDefFoundError: qupath/lib/gui/ActionTools
	at qupath.lib.extension.monailabel.Extension.installExtension(Extension.java:44)
	at qupath.lib.gui.ExtensionManager.refreshExtensions(ExtensionManager.java:148)
	at qupath.lib.gui.ExtensionManager.promptToCopyFilesToExtensionsDirectory(ExtensionManager.java:261)
	at qupath.lib.gui.viewer.DragDropImportListener.handleFileDropImpl(DragDropImportListener.java:334)
	at qupath.lib.gui.viewer.DragDropImportListener.handleFileDrop(DragDropImportListener.java:286)
	at qupath.lib.gui.viewer.DragDropImportListener.lambda$handle$0(DragDropImportListener.java:196)
	at com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:456)
	at java.base/java.security.AccessController.doPrivileged(Unknown Source)
	at com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:455)
	at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
	at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
	at com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:185)
	at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: qupath.lib.gui.ActionTools
	at java.base/java.net.URLClassLoader.findClass(Unknown Source)
	at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
	at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
	... 13 more

Start by reproducing the extension installation failure in QuPath 0.5.1 and inspect Extension.java, especially installExtension where the stack trace fails. Compare the extension's dependency usage with the QuPath 0.5.x API, then verify that the extension loads without the ActionTools NoClassDefFoundError.