Here is a set of models which triggers the NPE: https://git.opendaylight.org/gerrit/41029

And this is full NPE as reported by current Boron snapshot build:
[ERROR] yang-to-sources: Unable to generate sources with org.opendaylight.yangtools.maven.sal.api.gen.plugin.CodeGeneratorImpl generator
java.lang.NullPointerException
at org.opendaylight.yangtools.sal.binding.generator.impl.BindingGeneratorImpl.generateTypesFromAugmentedChoiceCases(BindingGeneratorImpl.java:1353)
at org.opendaylight.yangtools.sal.binding.generator.impl.BindingGeneratorImpl.augmentationToGenTypes(BindingGeneratorImpl.java:857)
at org.opendaylight.yangtools.sal.binding.generator.impl.BindingGeneratorImpl.allAugmentsToGenTypes(BindingGeneratorImpl.java:425)
at org.opendaylight.yangtools.sal.binding.generator.impl.BindingGeneratorImpl.generateTypes(BindingGeneratorImpl.java:262)
at org.opendaylight.yangtools.maven.sal.api.gen.plugin.CodeGeneratorImpl.generateSources(CodeGeneratorImpl.java:61)
at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.generateSourcesWithOneGenerator(YangToSourcesProcessor.java:348)
at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.generateSources(YangToSourcesProcessor.java:293)
at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.execute(YangToSourcesProcessor.java:95)
at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.conditionalExecute(YangToSourcesProcessor.java:104)
at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesMojo.execute(YangToSourcesMojo.java:119)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:207)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:307)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:193)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:106)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:863)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:288)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:199)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
[ERROR] yang-to-sources: One or more code generators failed, including failed list(generatorClass=exception)