Ensure swizzling in main thread (#557)

(cherry picked from commit ea3caea)

Author: nzagorchev

LeanplumSDK/LeanplumSDK/Classes/Internal/Leanplum.m

@@ -712,8 +712,6 @@ + (void)startWithUserId:(NSString *)userId
          userAttributes:(NSDictionary *)attributes
         responseHandler:(LeanplumStartBlock)startResponse
 {
-    [[Leanplum notificationsManager].proxy setupNotificationSwizzling];
-    
     if ([ApiConfig shared].appId == nil) {
         [self throwError:@"Please provide your app ID using one of the [Leanplum setAppId:] "
          @"methods."];
@@ -728,6 +726,8 @@ + (void)startWithUserId:(NSString *)userId
         return;
     }
 
+    [[Leanplum notificationsManager].proxy setupNotificationSwizzling];
+    
     [LPFileManager clearCacheIfSDKUpdated];
 
     [[LPCountAggregator sharedAggregator] incrementCount:@"start_with_user_id"];