[PLAT-9362] fix for data race and nullability issues

[kstenerud]

Goal

PLAT-9362

Fixed a data race when getting the host memory reading.

There were also warnings about incorrect nullability invariant expectations in the code, which are also now fixed.

Testing

Tested manually, reran all unit and e2e tests.

[github-actions]

Bugsnag.framework binary size increased by 152 bytes from 706,296 to 706,448

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%     +80  +0.1%     +80    Symbol Table
  +0.0%     +64  +0.0%     +64    String Table
  +0.0%     +16  +0.0%     +16    __TEXT,__text
  +0.5%      +8  +0.5%      +8    Function Start Addresses
  +0.1%      +4  +0.1%      +4    __TEXT,__unwind_info
  -0.1%     -20  -0.1%     -20    [__TEXT]
  [ = ]       0  -1.0%    -152    [__LINKEDIT]
  +0.0%    +152  [ = ]       0    TOTAL

Generated by 🚫 Danger

[lemnik]

LGTM