split layout can't be extended

[UrsKipfer]

I tried to extend the element but then I get an Error/Warning.
class MySplitLayout extends Vaadin.SplitLayoutElement {
static get is() { return 'my-split-layout'; }
........

I get: Could not find style data in module named my-split-layout-default-theme

The problem seems to be the line 17 of themable-mixin because this.is is is now the name of my element.

[tomivirkki]

Thanks for reporting, fix in vaadin/vaadin-themable-mixin#18