On a read-only and mirror web, when a non existing topic is viewed, the page content is blank rather than WebTopicViewTemplate or WebTopicNonWikiTemplate. This is because WebTopicViewTemplate and WebTopicNonWikiTemplate contain links to create the topic on the spot, which is not possible on a read-only or mirror web.
However, displaying blank content in this situation is not a business of TWiki::UI::View::view(). It's something WebTopicViewTemplate and WebTopicNonWikiTemplate need to care about.
As such, lib/TWiki/UI/View.pm and data/TWiki/WebTopicViewTemplate need to be amended accordingly so that the "This topic does not exist" message is displayed even on a read-only web or a mirror web.