fix(issue#549): Fix error appears when trying to open finished courselet.
fix(issue#549): Fix error appears when trying to open finished courselet.

If chat has no last_modify_timestamp we should get last message and assign it's timestamp to this field.  Messageges should be filtered out to not include messages with timestamp NULL.

raccoongang/socraticqs2,cjlee112/socraticqs2,raccoongang/socraticqs2,raccoongang/socraticqs2,cjlee112/socraticqs2,raccoongang/socraticqs2,cjlee112/socraticqs2,cjlee112/socraticqs2