Repro:

=== P.java
import java.io.IOException;
import java.io.OutputStream;
import java.io.UncheckedIOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.element.TypeElement;
import javax.tools.JavaFileObject;

@SupportedAnnotationTypes("*")
public class P extends AbstractProcessor {

  boolean first = true;

  @Override
  public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    if (first) {
      try {
        JavaFileObject jfo =
            processingEnv
                .getFiler()
                .createClassFile("T", roundEnv.getRootElements().iterator().next());
        try (OutputStream out = jfo.openOutputStream()) {
          Files.copy(Paths.get("T.class"), out);
        }
      } catch (IOException e) {
        throw new UncheckedIOException(e);
      }
      try {
        JavaFileObject jfo =
            processingEnv
                .getFiler()
                .createClassFile("T$1", roundEnv.getRootElements().iterator().next());
        try (OutputStream out = jfo.openOutputStream()) {
          Files.copy(Paths.get("T$1.class"), out);
        }
      } catch (IOException e) {
        throw new UncheckedIOException(e);
      }
      first = false;
    }
    return false;
  }
}
=== T.java
class T {
  void f() {
    new Object() {};
  }
}
=== Test.java
class Test {}
===

$ javac T.java P.java
$ javac -processor P Test.java -d out
...
An exception has occurred in the compiler (11-ea). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program and the following diagnostic in your report. Thank you.
java.lang.NullPointerException
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.enterClassFiles(JavacProcessingEnvironment.java:1182)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.<init>(JavacProcessingEnvironment.java:1076)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.next(JavacProcessingEnvironment.java:1100)
        at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(JavacProcessingEnvironment.java:1332)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:1258)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:936)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:311)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:170)
        at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:57)
        at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:43)