Switching kernels in a notebook with tabledisplay causes a runtime error.

If a notebook has a BeakerX tabledisplay in it and the user switches kernels, the keydown handler for tabledisplay functionality causes a failure here: https://github.com/twosigma/beakerx/blob/bef44325c1b98c813a4efd244e15e7ec2f84fe03/js/lab/src/plugin/index.ts#L83

As a general principle, if we can remove the need for `window.beakerx` in favor of a private module variable, it would be better practice, but that is orthogonal to this specific bug.

Thank you!