ConversationSettings must retain the gallery view

// FREEBIE

Signal/src/ViewControllers/ThreadSettings/OWSConversationSettingsViewController.m

@@ -44,7 +44,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property (nonatomic) NSArray<NSNumber *> *disappearingMessagesDurations;
 @property (nonatomic) OWSDisappearingMessagesConfiguration *disappearingMessagesConfiguration;
-
+@property (nullable, nonatomic) MediaGalleryViewController *mediaGalleryViewController;
 @property (nonatomic, readonly) TSAccountManager *accountManager;
 @property (nonatomic, readonly) OWSContactsManager *contactsManager;
 @property (nonatomic, readonly) OWSMessageSender *messageSender;
@@ -1168,6 +1168,10 @@ NS_ASSUME_NONNULL_BEGIN
     MediaGalleryViewController *vc =
         [[MediaGalleryViewController alloc] initWithThread:self.thread uiDatabaseConnection:self.uiDatabaseConnection];
 
+    // although we don't present the mediaGalleryViewController directly, we need to maintain a strong
+    // reference to it until we're dismissed.
+    self.mediaGalleryViewController = vc;
+
     [vc pushTileViewFromNavController:self.navigationController];
 }
 #pragma mark - Notifications