the `NullTopic` class can serve as a surrogate to areas where querying for a topic returns nil. Instead of checking for nil, we can send the same messages to this "placeholder" class and not worry about that nil check.  For example, see here for where I use it: https://github.com/jayroh/thredded_gem/pull/14/files#diff-7d67a236abe1bf19d5c5f278250a412dR47