Update RNFetchBlob.m

Solved an issue where the rootNavigationController is not necessarily the currently displayed controller, which caused an error of:
Warning: Attempt to present <SecondViewController: 0x7fb54b523240> on <ViewController: 0x7fb54b61e7f0> whose view is not in the window hierarchy!

ios/RNFetchBlob/RNFetchBlob.m

@@ -622,7 +622,12 @@ - (NSDictionary *)constantsToExport
 - (UIViewController *) documentInteractionControllerViewControllerForPreview: (UIDocumentInteractionController *) controller
 {
     UIWindow *window = [UIApplication sharedApplication].keyWindow;
-    return window.rootViewController;
+    UIViewController *currentlyPresentedView = [window.rootViewController presentedViewController];
+    if (currentlyPresentedView == nil)
+    {
+        return window.rootViewController;
+    }
+    return currentlyPresentedView;
 }
 
 # pragma mark - check expired network events