Fix crash when attaching media

Seems these crashes were introduced when transitioning from
DJWActionSheet -> UIAlertController

// FREEBIE

Signal/src/view controllers/MessagesViewController.m

@@ -1625,7 +1625,9 @@ typedef enum : NSUInteger {
         picker.mediaTypes = @[ (__bridge NSString *)kUTTypeImage, (__bridge NSString *)kUTTypeMovie ];
         picker.allowsEditing = NO;
         picker.delegate = self;
+        dispatch_async(dispatch_get_main_queue(), ^{
             [self presentViewController:picker animated:YES completion:[UIUtil modalCompletionBlock]];
+        });
     }
                    alertActionHandler:nil];
 }
@@ -1639,7 +1641,9 @@ typedef enum : NSUInteger {
     picker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
     picker.delegate = self;
     picker.mediaTypes = @[ (__bridge NSString *)kUTTypeImage, (__bridge NSString *)kUTTypeMovie ];
+    dispatch_async(dispatch_get_main_queue(), ^{
         [self presentViewController:picker animated:YES completion:[UIUtil modalCompletionBlock]];
+    });
 }
 
 /*
@@ -1755,6 +1759,7 @@ typedef enum : NSUInteger {
                                                  attachmentIds:[NSMutableArray new]];
     }
 
+    dispatch_async(dispatch_get_main_queue(), ^{
         [self dismissViewControllerAnimated:YES
                                  completion:^{
                                      DDLogVerbose(@"Sending attachment. Size in bytes: %lu, contentType: %@",
@@ -1771,6 +1776,7 @@ typedef enum : NSUInteger {
                                                                                    @"%@ Failed to send message attachment with error: %@", self.tag, error);
                                                                     }];
                                  }];
+    });
 }
 
 - (NSURL *)videoTempFolder {