Add sidebar
Avoid NPE in error controller
Add additional ViewResolver configuration

The DispatcherServlet adds a default InternalViewResolver
which was used by some apps, but when the actuator was
available it added an "/error" bean and effectively
switched off the default view resolver. The net fix was
to add an InternalViewResolver at the same time as
adding any other ViewResolvers.

[Fixes #55357516] [bs-290] Actuator UI app cannot serve static index.html