debugger: throw a more useful error when the frame is missing

PR-URL: nodejs#42776
Fixes: nodejs#42775
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/debugger/inspect_repl.js

@@ -635,6 +635,9 @@ function createRepl(inspector) {
 
   // List source code
   function list(delta = 5) {
+    if (!selectedFrame) {
+      throw new ERR_DEBUGGER_ERROR('Requires execution to be paused');
+    }
     return selectedFrame.list(delta).then(null, (error) => {
       print("You can't list source code right now");
       throw error;

test/sequential/test-debugger-list.js

@@ -0,0 +1,30 @@
+'use strict';
+const common = require('../common');
+
+common.skipIfInspectorDisabled();
+
+const fixtures = require('../common/fixtures');
+const startCLI = require('../common/debugger');
+
+const assert = require('assert');
+
+const cli = startCLI([fixtures.path('debugger/three-lines.js')]);
+
+(async () => {
+  await cli.waitForInitialBreak();
+  await cli.waitForPrompt();
+  await cli.command('list(0)');
+  assert.match(cli.output, /> 1 let x = 1;/);
+  await cli.command('list(1)');
+  assert.match(cli.output, /> 1 let x = 1;\r?\n {2}2 x = x \+ 1;/);
+  await cli.command('list(10)');
+  assert.match(cli.output, /> 1 let x = 1;\r?\n {2}2 x = x \+ 1;\r?\n {2}3 module\.exports = x;\r?\n {2}4 /);
+  await cli.command('c');
+  await cli.waitFor(/disconnect/);
+  await cli.waitFor(/debug> $/);
+  await cli.command('list()');
+  await cli.waitFor(/ERR_DEBUGGER_ERROR/);
+  assert.match(cli.output, /Uncaught Error \[ERR_DEBUGGER_ERROR\]: Requires execution to be paused/);
+})()
+.finally(() => cli.quit())
+.then(common.mustCall());