"-group" option issue for classes from default package

-group option issue for classes from default package: NPE is generated and then ignored, generated javadoc is incorrect (no .css, etc.)

java.lang.NullPointerException
at jdk.javadoc.internal.doclets.toolkit.util.Group.regExpGroupName(Group.java:217)
at jdk.javadoc.internal.doclets.toolkit.util.Group.groupPackages(Group.java:196)
at jdk.javadoc.internal.doclets.formats.html.PackageIndexWriter.<init>(PackageIndexWriter.java:91)
at jdk.javadoc.internal.doclets.formats.html.PackageIndexWriter.generate(PackageIndexWriter.java:104)
at jdk.javadoc.internal.doclets.formats.html.HtmlDoclet.generateOtherFiles(HtmlDoclet.java:149)
at jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:167)
at jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.startDoclet(AbstractDoclet.java:104)
at jdk.javadoc.internal.doclets.formats.html.HtmlDoclet.run(HtmlDoclet.java:83)
at jdk.javadoc.internal.doclets.standard.Standard.run(Standard.java:63)
at jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:393)
at jdk.javadoc.internal.tool.Start.begin(Start.java:272)
at jdk.javadoc.internal.tool.Start.begin(Start.java:219)
at jdk.javadoc.internal.tool.Main.execute(Main.java:85)
at JavadocTester.javadoc(JavadocTester.java:303)
at TestGroupOption.test2(TestGroupOption.java:60)
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:520)

Use attached test to reproduce.