Fix NullReferenceException when accessing f_builtins of a traced frame

src/core/IronPython/Runtime/Exceptions/TraceBack.cs

@@ -198,7 +198,9 @@ public FunctionCode f_code {
 
         public object f_builtins {
             get {
-                return _context.LanguageContext.BuiltinModuleDict;
+                // a frame created by the trace listener has no context of its own, but the listener knows the language context
+                PythonContext context = _context?.LanguageContext ?? _traceAdapter.PythonContext;
+                return context.BuiltinModuleDict;
             }
         }
 

tests/suite/modules/system_related/test_sys.py

@@ -329,6 +329,20 @@ def test_settrace_Xtracing_module_frame(self):
         out = subprocess.check_output([sys.executable, '-X:Tracing', '-c', code])
         self.assertEqual(out.strip(), b"['<module>', 'g']")
 
+    def test_settrace_f_builtins(self):
+        # The frame of a traced function has the same builtins as the frame of code that is not traced
+        import builtins
+        def g():
+            return sys._getframe(0).f_builtins
+
+        sys.settrace(lambda *args: None)
+        try:
+            traced = g()
+        finally:
+            sys.settrace(None)
+        self.assertIs(traced, builtins.__dict__)
+        self.assertIs(sys._getframe(0).f_builtins, builtins.__dict__)
+
     @skipUnlessIronPython()
     @unittest.expectedFailure # dummy getrefcount removed in https://github.com/IronLanguages/ironpython3/pull/577
     def test_getrefcount(self):