App crash

[Adrenocortico]

When my app goes in background, after a few seconds, it crashes and it comes up this log:

Assertion failed: (success), function __26-[MBLMetaWear synchronize]_block_invoke, file /Users/sschiffli/Code/metawear-ios-api/MetaWear/MBLMetaWear.m, line 850. (lldb)

`__26-[MBLMetaWear synchronize]_block_invoke:
0x100216884 <+0>: stp x29, x30, [sp, #-16]!
0x100216888 <+4>: mov x29, sp
0x10021688c <+8>: ldp x8, x2, [x0, #32]
0x100216890 <+12>: adrp x9, 506
0x100216894 <+16>: ldr x1, [x9, #3904]
0x100216898 <+20>: orr w3, wzr, #0x1
0x10021689c <+24>: mov x0, x8
0x1002168a0 <+28>: bl 0x1002f3928 ; symbol stub for: objc_msgSend
0x1002168a4 <+32>: tbz w0, #0, 0x1002168b0 ; <+44> at MBLMetaWear.m:850
0x1002168a8 <+36>: ldp x29, x30, [sp], #16
0x1002168ac <+40>: ret
0x1002168b0 <+44>: adrp x0, 306
0x1002168b4 <+48>: add x0, x0, #3324 ; =3324
0x1002168b8 <+52>: adrp x1, 306
0x1002168bc <+56>: add x1, x1, #2468 ; =2468
0x1002168c0 <+60>: adrp x3, 291
0x1002168c4 <+64>: add x3, x3, #2592 ; =2592
0x1002168c8 <+68>: movz w2, #0x352
0x1002168cc <+72>: bl 0x1002f3b20 ; symbol stub for: __assert_rtn