Fix crash when displaying UIAlertController for a test on device message that errored (#4446)

* Use application delegate window to show test on device error alert, run on main thread

* Refactor the display of the test on device error alert to use a window on the foregrounded scene

* More descriptive variable name for the view controller used to present the error alert

* Import UIKit at the top of FIRIAMDisplayExecutor.m

* Early return in test on device error flow if there's no foregrounded scene

* scripts/styles.sh

Author: christibbs

Firebase/InAppMessaging/Flows/FIRIAMDisplayExecutor.m

@@ -15,6 +15,7 @@
  */
 
 #import <FirebaseCore/FIRLogger.h>
+#import <UIKit/UIKit.h>
 
 #import "FIRCore+InAppMessaging.h"
 #import "FIRIAMActivityLogger.h"
@@ -41,6 +42,8 @@ @interface FIRIAMDisplayExecutor () <FIRInAppMessagingDisplayDelegate>
 @property(nonatomic) BOOL impressionRecorded;
 @property(nonatomic, nonnull, readonly) id<FIRIAMAnalyticsEventLogger> analyticsEventLogger;
 @property(nonatomic, nonnull, readonly) FIRIAMActionURLFollower *actionURLFollower;
+// Used for displaying the test on device message error alert.
+@property(nonatomic, strong) UIWindow *alertWindow;
 @end
 
 @implementation FIRIAMDisplayExecutor {
@@ -307,14 +310,37 @@ - (void)displayMessageLoadError:(NSError *)error {
 
   UIAlertAction *defaultAction = [UIAlertAction actionWithTitle:@"OK"
                                                           style:UIAlertActionStyleDefault
-                                                        handler:^(UIAlertAction *action){
+                                                        handler:^(UIAlertAction *action) {
+                                                          self.alertWindow.hidden = NO;
+                                                          self.alertWindow = nil;
                                                         }];
 
   [alert addAction:defaultAction];
 
-  [[UIApplication sharedApplication].keyWindow.rootViewController presentViewController:alert
-                                                                               animated:YES
-                                                                             completion:nil];
+  dispatch_async(dispatch_get_main_queue(), ^{
+#if defined(__IPHONE_13_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 130000
+    if (@available(iOS 13.0, *)) {
+      UIWindowScene *foregroundedScene = nil;
+      for (UIWindowScene *connectedScene in [UIApplication sharedApplication].connectedScenes) {
+        if (connectedScene.activationState == UISceneActivationStateForegroundActive) {
+          foregroundedScene = connectedScene;
+          break;
+        }
+      }
+
+      if (foregroundedScene == nil) {
+        return;
+      }
+      self.alertWindow = [[UIWindow alloc] initWithWindowScene:foregroundedScene];
+    }
+#else  // defined(__IPHONE_13_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 130000
+    self.alertWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
+#endif
+    UIViewController *alertViewController = [[UIViewController alloc] init];
+    self.alertWindow.rootViewController = alertViewController;
+    self.alertWindow.hidden = NO;
+    [alertViewController presentViewController:alert animated:YES completion:nil];
+  });
 }
 
 - (instancetype)initWithInAppMessaging:(FIRInAppMessaging *)inAppMessaging