Fix access to currentUserNotificationSettings from non-ui threads

Fixes #826

Author: LinusU

ios/RCTOneSignal/RCTOneSignalEventEmitter.m

@@ -121,16 +121,19 @@ + (void)sendEventWithName:(NSString *)name withBody:(NSDictionary *)body {
         return;
     }
 
-    NSUInteger types = 0;
-    if ([UIApplication instancesRespondToSelector:@selector(currentUserNotificationSettings)]) {
-        types = [RCTSharedApplication() currentUserNotificationSettings].types;
-    } else {
+    __block NSUInteger types = 0;
+
+    dispatch_sync(dispatch_get_main_queue(), ^{
+        if ([UIApplication instancesRespondToSelector:@selector(currentUserNotificationSettings)]) {
+            types = [RCTSharedApplication() currentUserNotificationSettings].types;
+        } else {
 
 #if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_8_0
-        types = [RCTSharedApplication() enabledRemoteNotificationTypes];
+            types = [RCTSharedApplication() enabledRemoteNotificationTypes];
 #endif
 
-    }
+        }
+    });
 
     callback(@[@{
        @"alert": @((types & UIUserNotificationTypeAlert) > 0),