Overriding the command registry keydown handler causes shortcut execution errors.

[afshin]

There is a spot where we monkey-patch the Lumino CommandRegistry's processKeydownEvent method and it is occasionally brittle, causing command invocations to fail. This seems like an anti-pattern for two reasons:

1. We are overriding the entire application's command registry and can cause other extensions to fail.
2. We are relying on an object we've attached to the window object instead of a local variable.

app.commands.processKeydownEvent = (event) => {
  if (window.beakerx.tableFocused) {
    return false;
  }

  return originalProcessFn.call(app.commands, event);
};

https://github.com/twosigma/beakerx_widgets/blob/master/beakerx_widgets/js/src/lab/BeakerxWidgetExtension.ts#L67-L69

This problem is sometimes triggered when a user changes the kernel on an active notebook multiple times. One user reported that after switching the kernel a second or third time, the window.beakerx object is undefined so the handler fails while trying to access the tableFocused attribute.

[afshin]

We should author a test that captures failures of window.beakerx as well ... unless we refactor it out entirely.