Fix freeze in host app after "dismissing" select thread VC

We must call these completion handlers whenever we exit the SAE

// FREEBIE

SignalMessaging/attachments/SharingThreadPickerViewController.m

@@ -171,6 +171,13 @@ typedef void (^SendMessageBlock)(SendCompletionBlock completion);
     }
 }
 
+// override
+- (void)dismissPressed:(id)sender
+{
+    DDLogDebug(@"%@ tapped dismiss share button", self.logTag);
+    [self cancelShareExperience];
+}
+
 - (void)didTapCancelShareButton
 {
     DDLogDebug(@"%@ tapped cancel share button", self.logTag);