Comparing with non-existing tag throws 500 error

Created by: demisx

The 7.1.1 seems to have more issues than the previous versions. At least, we notice more error pages than before. This time, comparing an existing tag to a non-existing (e.g. https://git.host.com/sites/site1-com-desktop/compare/v6.15...v6.14 where tag v6.14 doesn't exist) throws a 500 server error. Here is the exception written to the log:

NoMethodError (undefined method `id' for nil:NilClass):
  app/controllers/projects/compare_controller.rb:15:in `show'