Null child action in group

I'm getting this error in my Rubymine logs when I try to use a plugin I have developed.

This only occurs when I use the plugin under Rubymine on Linux (it was build in IDEA on Windows, and runs fine in Rubymine on Windows):

2016-08-08 11:22:35,042 [ 16162] ERROR - ctionSystem.DefaultActionGroup - Null child action in group Tools () of class class com.intellij.openapi.actionSystem.DefaultCompactActionGroup, id=com.sage.ad.plugins.COABuilder
java.lang.Throwable
at com.intellij.openapi.diagnostic.Logger.error(Logger.java:127)
at com.intellij.openapi.actionSystem.DefaultActionGroup.unStub(DefaultActionGroup.java:356)
at com.intellij.openapi.actionSystem.DefaultActionGroup.getChildren(DefaultActionGroup.java:312)
at com.intellij.openapi.actionSystem.impl.Utils.expandActionGroup(Utils.java:125)
at com.intellij.openapi.actionSystem.impl.Utils.expandActionGroup(Utils.java:86)
at com.intellij.openapi.actionSystem.impl.Utils.fillMenu(Utils.java:280)
at com.intellij.openapi.actionSystem.impl.ActionMenu.f(ActionMenu.java:324)
at com.intellij.openapi.actionSystem.impl.ActionMenu.access$600(ActionMenu.java:51)
at com.intellij.openapi.actionSystem.impl.ActionMenu$MenuListenerImpl.menuSelected(ActionMenu.java:278)
at javax.swing.JMenu.fireMenuSelected(JMenu.java:1038)
at javax.swing.JMenu$MenuChangeListener.stateChanged(JMenu.java:1118)
at javax.swing.DefaultButtonModel.fireStateChanged(DefaultButtonModel.java:349)
at javax.swing.DefaultButtonModel.setSelected(DefaultButtonModel.java:232)
at javax.swing.JMenu.setSelected(JMenu.java:305)
at javax.swing.JMenu.menuSelectionChanged(JMenu.java:1187)
at com.intellij.openapi.actionSystem.impl.ActionMenu.menuSelectionChanged(ActionMenu.java:240)
at javax.swing.MenuSelectionManager.setSelectedPath(MenuSelectionManager.java:125)
at javax.swing.plaf.basic.BasicMenuUI$Handler.mouseEntered(BasicMenuUI.java:526)
at java.awt.Component.processMouseEvent(Component.java:6542)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.retargetMouseEnterExit(Container.java:4676)
at java.awt.LightweightDispatcher.trackMouseEnterExit(Container.java:4654)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4505)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at com.intellij.ide.IdeEventQueue.d(IdeEventQueue.java:863)
at com.intellij.ide.IdeEventQueue._dispatchEvent(IdeEventQueue.java:695)
at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.java:385)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

On Windows I've setup my dev env according to the tutorials, and am building with JDK1.8.0_91.

On Linux Rubymine is running on JDK1.8.0_76. I would hope these are close enough to not make any difference...?

I've found a couple of other posts about this, but they don't seem too relevant to my case?