"...is it safe to port forward?"
Yes, providing you set up the rules in your router firewall correctly.
The unknowns are the app and the DVR. Just how secure are they? I don't know and I suspect you don't either.
Personally, unless there is a compelling reason to open up inbound ports, I wouldn't. Why risk it? However, if it's a commercial grade system incorporating secure connections, you probably don't have much to worry about.
It's a question of balance. If the data on your NAS drives is valuable and remote access isn't a priority, the answer should be obvious.
As a general rule, only let through your network what is absolutely essential. It's all about reducing and managing risk.