ThemeManager fails when removing a theme

Steps to reproduce

• Add a theme to your Integrated installation
• Assign the theme to a website
• Add a page to the website
• Remove the theme from your Integrated installation
• Result: exception on all pages of the channel. In this case the default theme should be used automatically

InvalidArgumentException in ThemeManager.php line 122:
Theme "thompson" not exists.
in ThemeManager.php line 122
at ThemeManager->setActiveTheme('thompson') in WebsiteConfigurationListener.php line 91
at WebsiteConfigurationListener->onKernelRequest(object(GetResponseEvent), 'kernel.request', object(TraceableEventDispatcher))
at call_user_func(array(object(WebsiteConfigurationListener), 'onKernelRequest'), object(GetResponseEvent), 'kernel.request', object(TraceableEventDispatcher)) in WrappedListener.php line 61
at WrappedListener->__invoke(object(GetResponseEvent), 'kernel.request', object(TraceableEventDispatcher))
at call_user_func(object(WrappedListener), object(GetResponseEvent), 'kernel.request', object(TraceableEventDispatcher)) in WrappedListener.php line 61
at WrappedListener->__invoke(object(GetResponseEvent), 'kernel.request', object(ContainerAwareEventDispatcher))

Requirements

• Removing a theme or removing a base view from a theme shouldn't throw an exception, but should fallback to the default theme, default base view
• Don't store the full path of a view, so changing a theme should affect all pages automatically