Makes dynamic lookup for the top view controller in order to present image or camera picker.

Existing implementation is incorrect especially when adding project as a module with more complex presentation structure.
If window's root view controller has presented modally any other view controller, it cannot present any other any more. Only the top most controller can present other.

With this change, the image picker/camera controller is presented on the top most view controller.

Author: chris-rutkowski

AUTHORS

@@ -56,4 +56,5 @@ Giancarlo Rocha <giancarloiff@gmail.com>
 Ryo Miyake <ryo@miyake.id>
 Théo Champion <contact.theochampion@gmail.com>
 Kazuki Yamaguchi <y.kazuki0614n@gmail.com>
-Eitan Schwartz <eshvartz@gmail.com>
+Eitan Schwartz <eshvartz@gmail.com>
+Chris Rutkowski <chrisrutkowski89@gmail.com>

packages/image_picker/image_picker/ios/Classes/FLTImagePickerPlugin.m

@@ -25,33 +25,29 @@ @interface FLTImagePickerPlugin () <UINavigationControllerDelegate, UIImagePicke
 @implementation FLTImagePickerPlugin {
   NSDictionary *_arguments;
   UIImagePickerController *_imagePickerController;
-  UIViewController *_viewController;
   UIImagePickerControllerCameraDevice _device;
 }
 
 + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar {
   FlutterMethodChannel *channel =
       [FlutterMethodChannel methodChannelWithName:@"plugins.flutter.io/image_picker"
                                   binaryMessenger:[registrar messenger]];
-  UIViewController *viewController =
-      [UIApplication sharedApplication].delegate.window.rootViewController;
-  FLTImagePickerPlugin *instance =
-      [[FLTImagePickerPlugin alloc] initWithViewController:viewController];
+  FLTImagePickerPlugin *instance = [FLTImagePickerPlugin new];
   [registrar addMethodCallDelegate:instance channel:channel];
 }
 
-- (instancetype)initWithViewController:(UIViewController *)viewController {
-  self = [super init];
-  if (self) {
-    _viewController = viewController;
-  }
-  return self;
-}
-
 - (UIImagePickerController *)getImagePickerController {
   return _imagePickerController;
 }
 
+- (UIViewController *)viewController {
+  UIViewController *topController = [UIApplication sharedApplication].keyWindow.rootViewController;
+  while (topController.presentedViewController) {
+    topController = topController.presentedViewController;
+  }
+  return topController;
+}
+
 - (void)handleMethodCall:(FlutterMethodCall *)call result:(FlutterResult)result {
   if (self.result) {
     self.result([FlutterError errorWithCode:@"multiple_request"
@@ -136,7 +132,7 @@ - (void)showCamera {
       [UIImagePickerController isCameraDeviceAvailable:_device]) {
     _imagePickerController.sourceType = UIImagePickerControllerSourceTypeCamera;
     _imagePickerController.cameraDevice = _device;
-    [_viewController presentViewController:_imagePickerController animated:YES completion:nil];
+    [[self viewController] presentViewController:_imagePickerController animated:YES completion:nil];
   } else {
     [[[UIAlertView alloc] initWithTitle:@"Error"
                                 message:@"Camera not available."
@@ -241,7 +237,7 @@ - (void)errorNoPhotoAccess:(PHAuthorizationStatus)status {
 - (void)showPhotoLibrary {
   // No need to check if SourceType is available. It always is.
   _imagePickerController.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
-  [_viewController presentViewController:_imagePickerController animated:YES completion:nil];
+  [[self viewController] presentViewController:_imagePickerController animated:YES completion:nil];
 }
 
 - (void)imagePickerController:(UIImagePickerController *)picker