title: Show error [NSConditionLock lockWhenCondition:beforeDate:] when call background methods iOS
body: <p>When I run multiple methods in the background, my app sometimes shows the error below. It says it cannot execute the background methods.</p>
  
  <pre><code>[NSConditionLock lockWhenCondition:beforeDate:]: deadlock (&lt;NSConditionLock: 0x166ae1a0&gt; '(null)')
   *** Break on _NSLockError() to debug.
  </code></pre>
  
  <p>Why is my app getting this error? How can I solve this issue?</p>