Renderer: fix blur error after removing focused widget

Summary:
If you focused a widget in the renderer, then changed the editor
contents, then focused a new widget, a js error would be thrown trying
to blur the previous widget, which no longer existed. We now check in
blur for the widget existing so that we just allow the blur to go
through.

It might make more sense later on to keep track of when a widget
disappears through a rerender and check for the currentFocus at that
point. But currently we don't really need this.

Test plan:
Make a grapher, then change it to a number-line, and drag the point.
Make sure no js error is thrown.

Auditors: michelle