Just today I sent this fix to FMP's author but you are welcome to use the fix:
I built a 64-bit version using Xcode 8.3.2 on macOS Sierra.
In file FPTextAreaB.m and this method, - (NSDictionary *)archivalDictionary
old code:
NSData *d = [_textStorage RTFFromRange:NSMakeRange(0, [_textStorage length]) documentAttributes:nil];
new code:
NSData *d = [_textStorage RTFFromRange:NSMakeRange(0, [_textStorage length])
documentAttributes:@{NSDocumentTypeDocumentAttribute: NSRTFTextDocumentType}]; //Brian 20170720
I built a 64-bit version using Xcode 8.3.2 on macOS Sierra.
The only other change is in Build Settings for the target:

Best Regards,
Brian, BrilorSoftware
Developer of PropertyMaintenanceTracker, VehicleMaintenanceTracker, PPCToTrash, CCNX, SDT, co-developer FutureBASIC and other Macintosh shareware and freeware.