Change the status bar to white when MFMailComposeViewController is launched from UIActivityViewController on iOS 11.

MFMailComposeViewController+XXAppearance.m

@implementation MFMailComposeViewController (XXAppearance)

// called when UIViewControllerBasedStatusBarAppearance = YES
- (UIStatusBarStyle)preferredStatusBarStyle
{
    return UIStatusBarStyleLightContent;
}

// called when UIViewControllerBasedStatusBarAppearance = YES
- (UIViewController *)childViewControllerForStatusBarStyle
{
    return nil;
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
}

@end