APP CRASH : *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDictionaryM _fastCStringContents:]: unrecognized selector sent to instance

[NSManagedObjectContext(MagicalSaves) MR_saveWithOptions:completion:]_block_invoke(0x283728f00) → Finished saving: <NSManagedObjectContext (0x283728f00): *** BACKGROUND SAVING (ROOT) ***> on *** MAIN THREAD ***
2019-04-05 13:12:02.254708+0530 AppName[7746:2251556] Received
2019-04-05 13:12:02.255348+0530 AppName[7746:2251556] -[__NSDictionaryM _fastCStringContents:]: unrecognized selector sent to instance 0x28088a5c0
2019-04-05 13:12:02.255610+0530 AppName[7746:2251556] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDictionaryM _fastCStringContents:]: unrecognized selector sent to instance 0x28088a5c0'
*** First throw call stack:
(0x1b9c77ea0 0x1b8e49a40 0x1b9b90d04 0x1b9c7d7b8 0x1b9c7f45c 0x1b98b4d64 0x1b9c67e2c 0x1ba70e390 0x1ba70e3c4 0x1007348e0 0x103317158 0x104cc7824 0x104cc8dc8 0x104cd6a78 0x1b9c07dd0 0x1b9c02c98 0x1b9c021cc 0x1bbe79584 0x1e6e45054 0x1008ff9d0 0x1b96c2bb4)
libc++abi.dylib: terminating with uncaught exception of type NSException

Please help me with this or trouble shoot it.

MagicalRecord doesn't use _fastCStringContents: directly. So your issue is lacking a Minimal, Reproducible Example.

You'd need to share a piece of code that can reproduce that issue, and you'll need to specify the environment (iOS, macOS?). Please do so and we'll re-open the issue.

[edit]
Also, the only line of code that comes after the log of Finished saving: is:

MagicalRecord/MagicalRecord/Categories/NSManagedObjectContext/NSManagedObjectContext+MagicalSaves.m

Lines 118 to 120 in 9417316

    
           dispatch_async(dispatch_get_main_queue(), ^{ 
        
               completion(saveResult, error); 
        
           });

So your issue depends on whatever you have in your completion block.

	dispatch_async(dispatch_get_main_queue(), ^{
	completion(saveResult, error);
	});