Can't loading plugin with EmptyThrowable

I've created a simple pulgin for the Intellij IDEA and Android Studio, it work fine in most of the time, but in some user's IDE the crash occuring:

EmptyThrowable: Null child action in group Main Toolbar () of class class com.intellij.openapi.actionSystem.DefaultActionGroup, id=Moxun.generator

in plugin.xml, I add plugin to group "MainToolBar", is the bug caused by this?
my develop environment is JDK1.8 and IDEA 14, project language level is 6.
project code here:

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