Flutter web - why it is getting 404 error

Failed to load resource: the server responded with a status of 404 () (index):50 Uncaught ReferenceError: _flutter is not defined at (index):50:7

  <script>
window.addEventListener('load', function(ev) {
  // Download main.dart.js
  _flutter.loader.loadEntrypoint({
    serviceWorker: {
      serviceWorkerVersion: serviceWorkerVersion,
    }
  }).then(function(engineInitializer) {
    return engineInitializer.initializeEngine();
  }).then(function(appRunner) {
    return appRunner.runApp();
  });
});