Segfault when Sidebar has no child

[jon-dez]

There is a segfault when borealis tries to render a sidebar with no children.
Unsure if this is intended, but a quick fix is trivial.

// File: library/lib/sidebar.cpp

View* Sidebar::getDefaultFocus()
{

    ...

// Line 43
    View* toFocus{ nullptr };
    if(this->children.size() != 0)
        // Try to focus last focused one
        toFocus = this->children[this->lastFocus]->view->getDefaultFocus();

    ...

}