NPE at com.vaadin.data.fieldgroup.FieldGroup.commit

[thehappycoder]

When I click the submit button, I get

SEVERE: 
com.vaadin.server.ServerRpcManager$RpcInvocationException: Unable to invoke method click in com.vaadin.shared.ui.button.ButtonServerRpc
    at com.vaadin.server.ServerRpcManager.applyInvocation(ServerRpcManager.java:170)
    at com.vaadin.server.ServerRpcManager.applyInvocation(ServerRpcManager.java:118)
    at com.vaadin.server.communication.ServerRpcHandler.handleBurst(ServerRpcHandler.java:207)
    at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:111)
    at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:91)
    at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:37)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1390)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:238)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:711)
    at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:552)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
    at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:568)
    at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:221)
    at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1112)
    at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:479)
    at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:183)
    at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1046)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
    at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:199)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
    at org.eclipse.jetty.server.Server.handle(Server.java:462)
    at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:281)
    at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:232)
    at org.eclipse.jetty.io.AbstractConnection$1.run(AbstractConnection.java:505)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:607)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:536)
    at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.vaadin.server.ServerRpcManager.applyInvocation(ServerRpcManager.java:168)
    ... 27 more
Caused by: com.vaadin.event.ListenerMethod$MethodException: Invocation of method buttonClick in vaadin.scala.internal.ButtonClickListener failed.
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:528)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:198)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:161)
    at com.vaadin.server.AbstractClientConnector.fireEvent(AbstractClientConnector.java:969)
    at com.vaadin.ui.Button.fireClick(Button.java:368)
    at com.vaadin.ui.Button$1.click(Button.java:57)
    ... 32 more
Caused by: java.lang.NullPointerException
    at com.vaadin.data.fieldgroup.FieldGroup.commit(FieldGroup.java:442)
    at vaadin.scala.FieldGroup$$anonfun$commit$1.apply$mcV$sp(FieldGroup.scala:100)
    at vaadin.scala.FieldGroup$$anonfun$commit$1.apply(FieldGroup.scala:100)
    at vaadin.scala.FieldGroup$$anonfun$commit$1.apply(FieldGroup.scala:100)
    at scala.util.control.Exception$Catch$$anonfun$either$1.apply(Exception.scala:124)
    at scala.util.control.Exception$Catch$$anonfun$either$1.apply(Exception.scala:124)
    at scala.util.control.Exception$Catch.apply(Exception.scala:102)
    at scala.util.control.Exception$Catch.either(Exception.scala:124)
    at vaadin.scala.FieldGroup.commit(FieldGroup.scala:100)
    at co.cleancode.scaladin.FieldGroup.commit(FieldGroup.scala:33)
    at co.cleancode.menu.views.product.ProductManageWindow$Form$$anon$11$$anonfun$10.apply(ProductManageWindow.scala:189)
    at co.cleancode.menu.views.product.ProductManageWindow$Form$$anon$11$$anonfun$10.apply(ProductManageWindow.scala:188)
    at vaadin.scala.internal.ButtonClickListener.buttonClick(listeners.scala:84)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:508)
    ... 37 more

Here is the code:
https://gist.github.com/thehappycoder/b697586fd89ea6b0cff2

After trial and error I figured that categoryId may be the cause but I don't understand why. And I cannot debug because of this issue henrikerola/sbt-vaadin-plugin#10

[thehappycoder]

That's due to vaadin.scala.FieldGroup's buildAndBindMemberFields ignoring the result of bind(f, propertyId):
Failure(com.vaadin.data.fieldgroup.FieldGroup$BindException: A property with id categoryId was not found in the item)

[thehappycoder]

Seems that vaadin's version of buildAndBindMemberFields allows to skip the binding but scaladin version doesn't allow. My form has this categoryId field that isn't supposed to be saved to database.