I'm experimenting. Could you help me? I have a problem.

Question

I'm experimenting with an app I've made in Xcode. I have a button, that displays the social networks (on click) only if the user drops an image into the image view. Otherwise the button does nothing. I would like to display the alert saying "You have to drop an image into the app." or something like that. Could you help me?
Tiny part of the code is down there...
- (IBAction)shareToService:(id)sender {
    NSMutableArray *shareItems = [NSMutableArray arrayWithObject:[self.textView string]];
    NSImage *image = [self.imageView image];
    if (image) {
        [shareItems addObject:image];
        NSSharingServicePicker *sharingPicker = [[NSSharingServicePicker alloc] initWithItems:shareItems];
        sharingPicker.delegate = self;
        [sharingPicker showRelativeToRect:[self.shareButton bounds] ofView:self.shareButton preferredEdge:NSMaxYEdge];
    else{nil;};

Answer 1

Sorry, I misread that completely. Replace that else clause with something like this:
  NSAlert * alert = [[NSAlert alloc] init];
  [alert setMessageText: NSLocalizedString(@"Image Required", NULL)];
  [alert setAlertStyle: NSInformationalAlertStyle];
  [alert setInformativeText: NSLocalizedString(@"You have to drop an image into the app", NULL)];
  [alert runModal];