in order to avoid potential errors related to GWT RPC serialization,
a recent patch  introduced the requirement for no-arg constructor
in backend "common" module's classes.
(Even though we're planning to move away from GWT RPC to using REST API,
for the time being, this should prevent RPC related errors caused by
missing no-arg constructor, which are sometimes hard to trace.)
For a select few classes *not* involved in RPC communication, you
can add these in checkstyle-suppressions.xml file.