I'm sorry you've had trouble with your project! However, how do you know it's Construct 2 and not your computer hard disk failing? If there is a problem in Construct 2 obviously we urgently want to fix it as quickly as possible, but how do we tell that apart from your computer corrupting the data itself in this case? So far you're the only one to report this "file filled with NULL" case, so I don't know if it's a Construct 2 problem or not...
I strongly urge you to use Construct 2's built-in backup options. You would have lost at most one day's work if you had a daily backup to your existing Dropbox folder. Even if Construct 2 is absolutely perfect and never fails in any way, your computer's hardware, operating system or other software might, causing a problem with your project. So even if we made Construct 2 invulnerable to any problem at all, it's still highly recommended to set up a solid backup scheme!