Fix thread-safety assert when sending video attachments.

// FREEBIE

Signal/src/view controllers/MessagesViewController.m

@@ -56,6 +56,7 @@
 #import <SignalServiceKit/OWSFingerprintBuilder.h>
 #import <SignalServiceKit/OWSMessageSender.h>
 #import <SignalServiceKit/SignalRecipient.h>
+#import <SignalServiceKit/Threading.h>
 #import <SignalServiceKit/TSAccountManager.h>
 #import <SignalServiceKit/TSInvalidIdentityKeySendingErrorMessage.h>
 #import <SignalServiceKit/TSMessagesManager.h>
@@ -1965,6 +1966,7 @@ typedef enum : NSUInteger {
                  [self dismissViewControllerAnimated:YES
                                           completion:^{
                                               OWSAssert([NSThread isMainThread]);
+                                              
                                               [self sendMessageAttachment:attachment];
                                           }];
              }
@@ -1974,12 +1976,12 @@ typedef enum : NSUInteger {
 
 - (void)sendMessageAttachment:(SignalAttachment *)attachment
 {
-    OWSAssert([NSThread isMainThread]);
     // TODO: Should we assume non-nil or should we check for non-nil?
     OWSAssert(attachment != nil);
     OWSAssert(![attachment hasError]);
     OWSAssert([attachment mimeType].length > 0);
     
+    DispatchMainThreadSafe(^{
         TSOutgoingMessage *message;
         OWSDisappearingMessagesConfiguration *configuration =
         [OWSDisappearingMessagesConfiguration fetchObjectWithUniqueID:self.thread.uniqueId];
@@ -2009,6 +2011,7 @@ typedef enum : NSUInteger {
                                            DDLogError(
                                                       @"%@ Failed to send message attachment with error: %@", self.tag, error);
                                        }];
+    });
 }
 
 - (NSURL *)videoTempFolder {