title: Xcode build running on device but failing on simultor
body: <p>I'm using CocoaPods with a React Native app. I've had various errors when running builds in the Xcode simulator. I don't have issues running it on my device. Here's one that I get when I have breakpoints enabled:</p>
  
  <pre><code>- (void)ensureOnJavaScriptThread:(dispatch_block_t)block
      {
        RCTAssert(_jsThread, @"This method must not be called before the JS thread is created");
  </code></pre>
  
  <p>The error here is green (yet breaking, thanks Xcode) and it says <code>com.facebook.react.JavaScript (9): breakpoint 1.2</code></p>
  
  <p>I get two errors when I disable breakpoints, this is one:</p>
  
  <pre><code>void Instance::loadApplication(std::unique_ptr&lt;RAMBundleRegistry&gt; bundleRegistry,
                                     std::unique_ptr&lt;const JSBigString&gt; string,
                                     std::string sourceURL) {
        callback_-&gt;incrementPendingJSCalls();
        SystraceSection s("Instance::loadApplication", "sourceURL",
                          sourceURL);
        nativeToJsBridge_-&gt;loadApplication(std::move(bundleRegistry), std::move(string),
                                           std::move(sourceURL));
      }
  </code></pre>
  
  <p>with the red error on the <code>callback_-&gt;</code> line reading <code>EXC_BAD_ACCESS (code=EXC_I386_GPFLT)</code></p>
  
  <p>the second is this a <code>signal SIGABRT</code> error from this code:</p>
  
  <pre><code>int main(int argc, char * argv[]) { 
    @autoreleasepool {
      return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
  }
  </code></pre>
  
  <p>I think i've fixed that issue before by enabling zombies. When I do that, I see this in the console:</p>
  
  <pre><code>2018-07-11 16:56:08.326 [info][tid:main][RCTRootView.m:293] Running application Mapp ({
  initialProps =     {
  };
  rootTag = 11;
  })
  =================================================================
  Main Thread Checker: UI API called on a background thread: -[UIApplication setNetworkActivityIndicatorVisible:]
  PID: 34682, TID: 5546845, Thread name: (none), Queue name: com.mixpanel.20e6d2c2b6c431dfecfdfaa100ec0a11.0x7fa94db06940.network, QoS: 0
  Backtrace:
  4   Mapp                           0x0000000103a31dd9 -[MPNetwork updateNetworkActivityIndicator:] + 121
  5   Mapp                           0x0000000103a2ec09 -[MPNetwork flushQueue:endpoint:] + 985
  6   Mapp                           0x0000000103a2e7b0 -[MPNetwork flushEventQueue:] + 64
  7   Mapp                           0x0000000103a10418 __32-[Mixpanel flushWithCompletion:]_block_invoke + 312
  8   libdispatch.dylib                   0x000000010e3f47ab _dispatch_call_block_and_release + 12
  9   libdispatch.dylib                   0x000000010e3f57ec _dispatch_client_callout + 8
  10  libdispatch.dylib                   0x000000010e3fdbe5 _dispatch_queue_serial_drain + 1305
  11  libdispatch.dylib                   0x000000010e3fe4fa _dispatch_queue_invoke + 328
  12  libdispatch.dylib                   0x000000010e3fa344 _dispatch_queue_override_invoke + 726
  13  libdispatch.dylib                   0x000000010e40136c _dispatch_root_queue_drain + 664
  14  libdispatch.dylib                   0x000000010e401076 _dispatch_worker_thread3 + 132
  15  libsystem_pthread.dylib             0x000000010e920169 _pthread_wqthread + 1387
  16  libsystem_pthread.dylib             0x000000010e91fbe9 start_wqthread + 13
  </code></pre>
  
  <p>This to me is unreadable. I don't understand what's wrong. Reminder: this build runs without breaking errors on my device, even when breakpoints are enabled. Is this something I should worry about? Please help!</p>