Filevault Decryption process : Kernel Panic - Method to kill decryption process?
Unfortunately there's no way of stopping the decryption process.
The error message you get indicate that either your file system is corrupt, or there's a bug in Apple's APFS implementation.
You can try downgrading and/or upgrading macOS (if a newer version is available). This might fix the problem as you could possibly work around a possible bug.
Apart from that, the only other possible fix is to copy all your files to a different disk. Then you can format this drive from scratch, and copy your files back.