NPE is thrown which is caused by completions in extends clauses of nested classes:

-> class C {class D extends [TAB]

Exception in thread "main" java.lang.NullPointerException
        at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:890)
        at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:836)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:591)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:660)
        at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4356)
        at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4263)
        at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4192)
        at com.sun.tools.javac.comp.Attr.visitClassDef(Attr.java:883)
        at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:744)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:591)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:660)
        at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:679)
        at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1135)
        at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:984)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:591)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:660)
        at com.sun.tools.javac.comp.Attr.attribStatToTree(Attr.java:409)
        at com.sun.tools.javac.api.JavacTrees.attribStatToTree(JavacTrees.java:836)
        at com.sun.tools.javac.api.JavacTrees.getAttrContext(JavacTrees.java:810)
        at com.sun.tools.javac.api.JavacTrees.getScope(JavacTrees.java:705)
        at com.sun.tools.javac.api.JavacTrees.getScope(JavacTrees.java:122)
        at jdk.jshell.SourceCodeAnalysisImpl.createAccessibilityFilter(SourceCodeAnalysisImpl.java:470)
        at jdk.jshell.SourceCodeAnalysisImpl.computeSuggestions(SourceCodeAnalysisImpl.java:243)
        at jdk.jshell.SourceCodeAnalysisImpl.completionSuggestions(SourceCodeAnalysisImpl.java:229)
        at jdk.internal.jshell.tool.ConsoleIOContext$2.complete(ConsoleIOContext.java:92)
        at jdk.internal.jline.console.ConsoleReader.complete(ConsoleReader.java:3290)
        at jdk.internal.jline.console.ConsoleReader.readLine(ConsoleReader.java:2650)
        at jdk.internal.jline.console.ConsoleReader.readLine(ConsoleReader.java:2277)
        at jdk.internal.jshell.tool.ConsoleIOContext.readLine(ConsoleIOContext.java:153)
        at jdk.internal.jshell.tool.JShellTool.run(JShellTool.java:447)
        at jdk.internal.jshell.tool.JShellTool.start(JShellTool.java:266)
        at jdk.internal.jshell.tool.JShellTool.start(JShellTool.java:245)
        at jdk.internal.jshell.tool.JShellTool.main(JShellTool.java:235)

These also could be the cause of the NPE:
-> class C {class D implements [TAB]
-> class C {interface I extends [TAB]

These aren't the cause of the NPE:
-> interface I {class C extends [TAB]
-> interface I {class C implements [TAB
-> interface I {interface I extends [TAB]