Closing last widget in tabbar

When closing last widget in tabbar focus is switched into last widget in previous tabbar.

The code responsible for that is here

I think that, it would be better to first check if prevBar.currentIndex is present and if so, use it in this.activateWidget.

Maybe somethink like this:

    activatePreviousTabBar(current: TabBar<Widget> | undefined = this.currentTabBar): boolean {
        const prevBar = this.previousTabBar(current);
        if (!prevBar) {
            return false;
        }
        if(!prevBar.currentIndex) {
            const len = prevBar.titles.length;
            prevBar.currentIndex = len - 1;
        }
        if (prevBar.currentTitle) {
            this.activateWidget(prevBar.currentTitle.owner.id);
        }
        return true;
    }

@psdutkiewicz if you believe it is an improvement, please open a corresponding issue, and feel free to also provide a pull-request!