Thats an easy place to fix. Could be a LOT worse.
Go to any auto parts store and grab a small fiberglass repair kit. Each of these should have small bits of fiberglass cloth that should be more than enough for such a fix.
Follow the instructions! It's very similar to doing paper mache'. Accept you have to sand the surface around the cracked area.
You soak the cloth in resin, then shape around the crack, let harden, sand down, and paint! EZ PZ.
I can't tell you the exact mixtures etc, but they are in the packages. Each is a tad different in detail, but the basic idea is still the same.
Allow time, it's messy, and wear gloves!
As a side note, I have had little "splits" along my door for years. It's one of those.. "I'll get around to it" things. Never had it effect anything and they haven't gotten any worse over the miles and miles I have towed.