[m-tab-bar] Errors after switching

[ghost]

Describe the bug
Console errors with m-tab-bar, possibly preventing from getting events from it.

To Reproduce
Reproducible on Codepen: https://codepen.io/antivitla/pen/qzVRwY
Try to switch tabs, check appearing console errors including 'Cannot read property 'getScrollPosition' of null"' and others inside material-components.

Screenshots
Screen from codepen:

Better errors when trying to import by module, this way error is happening inside MDCTabBar component code:

Desktop (please complete the following information):

• Windows 10, Chrome latest

[tychenjiajun]

Add a scroller and it will be fine https://codepen.io/anon/pen/ZNNqWw
I was too lazy to update the doc. 😂 But I'm also considering adding a built-in scroller to make it compatible with the 0.x.y versions.

[ghost]

Ah, great, thanks! :))