The TrackerControllers are only supposed to issue their queries once
all the pieces of the application that are necessary to formulate them
have been initialized. This is indicated by the invocation of the start
method. For example, if the queries are submitted before the detection
of the getting started PDF has completed, then they won't have the
path to PDF and it will be missing from the UI.

Throwing an exception forces one to think about the details surrounding
each query submission path, as opposed to having the silently dropping
the request.

https://bugzilla.gnome.org/show_bug.cgi?id=791518