A currently open PR exposed the NPE stack dump below. It can be reproduced by adding the following statement

import module jdk.httpserver
in src/jdk.httpserver/share/classes/module-info.java

Whether that is a valid statement is one question, but it shouldn't cause a javadoc NPE in this way
```
$ make docs
Building target 'docs' in configuration 'macosx-aarch64'
Compiling up to 58 files for jdk.httpserver
Creating ct.sym classes
Generating JDK_API javadoc for 57 modules
error: fatal error encountered: java.lang.NullPointerException: Cannot invoke "java.util.Set.contains(Object)" because "this.env.toplevel.modle.readModules" is null
error: Please file a bug against the javadoc tool via the Java bug reporting page
  (https://bugreport.java.com) after checking the Bug Database (https://bugs.java.com)
  for duplicates. Include error messages and the following diagnostic in your report. Thank you.
java.lang.NullPointerException: Cannot invoke "java.util.Set.contains(Object)" because "this.env.toplevel.modle.readModules" is null
at jdk.compiler.interim/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.doModuleImport(TypeEnter.java:452)
at jdk.compiler.interim/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.handleImports(TypeEnter.java:385)
at jdk.compiler.interim/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.resolveImports(TypeEnter.java:367)
at jdk.compiler.interim/com.sun.tools.javac.comp.TypeEnter.lambda$ensureImportsChecked$0(TypeEnter.java:160)
at jdk.compiler.interim/com.sun.tools.javac.comp.TypeEnter.finishImports(TypeEnter.java:212)
at jdk.compiler.interim/com.sun.tools.javac.comp.TypeEnter.ensureImportsChecked(TypeEnter.java:160)
at jdk.compiler.interim/com.sun.tools.javac.comp.Enter.complete(Enter.java:649)
at jdk.compiler.interim/com.sun.tools.javac.comp.Enter.main(Enter.java:601)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.JavadocEnter.main(JavadocEnter.java:72)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.JavadocTool.getEnvironment(JavadocTool.java:206)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:581)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.Start.begin(Start.java:399)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.Start.begin(Start.java:348)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.Main.execute(Main.java:57)
at jdk.javadoc.interim/jdk.javadoc.internal.tool.Main.main(Main.java:46)
2 errors
Docs.gmk:490: recipe for target '/Users/mimcmah/git/jdk/build/macosx-aarch64/support/docs/_javadoc_JDK_API_exec.marker' failed
make[3]: *** [/Users/mimcmah/git/jdk/build/macosx-aarch64/support/docs/_javadoc_JDK_API_exec.marker] Error 4
make/Main.gmk:529: recipe for target 'docs-jdk-api-javadoc' failed
make[2]: *** [docs-jdk-api-javadoc] Error 2
make[2]: *** Waiting for unfinished jobs....
```
This is with fully synced (as of today) jdk open and closed.