In the NPAPIJavaPlugin::NPAPIJavaPlugin constructor we are creating a PluginRunner instance to which the this* reference is passed (). At the end of the constructor the PluginRunner is started and the Jpi_PluginEvent_Init is pushed to the PluginRunner. It seems that sometimes PluginRunner starts working on its thread with non-completely constructed class instance. Normally this* instance shouldn't be published from constructor to another thread.
For more details see JDK-8021383