I don't have any problems with any of B4X, but for safety, I'm using https://www.b4x.com/android/forum/threads/svn-source-control-and-basic4android.10438/#content in combination with free backup tool (EaseUS Todo Backup Free, support incremental backups).
I have experience similar problem (4-5 years ago) with Visual Studio. From then, until now, I'm always using SVN and backups.
I deleted the metafile but no luck. The main project file has size 21kb but when I open it with notepad++ has only one line and says null null...
Very reluctantly i used a freeware "previous file recovery" http://www.nirsoft.net/utils/previous_files_recovery.html and...it worked!! Second time in my life that such a tool helped (the first time another recovery software restored a book a friend of mine wrote)
I don't know which parts of the code might be missing but from a first look it seems pretty solid.