Crash when saving read receipts

[stefanceriu]

Thread 22 name:
Thread 22 Crashed:
0   libobjc.A.dylib               	0x000000018a202e5c objc_retain_x0 + 16
1   Foundation                    	0x000000018b2d72d4 -[NSDictionary(NSDictionary) encodeWithCoder:] + 488 (NSDictionary_Foundation.m:46)
2   Foundation                    	0x000000018b2d3b44 _encodeObject + 1188 (NSKeyedArchiver.m:1234)
3   Foundation                    	0x000000018b31a1ec -[NSKeyedArchiver _encodeArrayOfObjects:forKey:] + 468 (NSKeyedArchiver.m:1279)
4   Foundation                    	0x000000018b2d732c -[NSDictionary(NSDictionary) encodeWithCoder:] + 576 (NSDictionary_Foundation.m:47)
5   Foundation                    	0x000000018b2d3b44 _encodeObject + 1188 (NSKeyedArchiver.m:1234)
6   Foundation                    	0x000000018b837474 +[NSKeyedArchiver archiveRootObject:toFile:] + 216 (NSKeyedArchiver.m:750)
7   MatrixSDK                     	0x0000000102bf9600 __27-[MXFileStore saveReceipts]_block_invoke + 460 (MXFileStore.m:2212)
8   libdispatch.dylib             	0x00000001984ac4b4 _dispatch_call_block_and_release + 32 (init.c:1518)
9   libdispatch.dylib             	0x00000001984adfdc _dispatch_client_callout + 20 (object.m:560)
10  libdispatch.dylib             	0x00000001984b5694 _dispatch_lane_serial_drain + 672 (inline_internal.h:2632)
11  libdispatch.dylib             	0x00000001984b61e0 _dispatch_lane_invoke + 384 (queue.c:3940)
12  libdispatch.dylib             	0x00000001984c0e10 _dispatch_workloop_worker_thread + 652 (queue.c:6846)
13  libsystem_pthread.dylib       	0x00000001ddf33df8 _pthread_wqthread + 288 (pthread.c:2618)
14  libsystem_pthread.dylib       	0x00000001ddf33b98 start_wqthread + 8