fix build for macOS 10.13

Change-Id: Ib45139875b3d58675ad7c3a477d2922463115033
diff --git a/src/MessagesVC.mm b/src/MessagesVC.mm
index 9a57e76..4e57319 100644
--- a/src/MessagesVC.mm
+++ b/src/MessagesVC.mm
@@ -179,7 +179,7 @@
         return cachedConv_;
     auto convOpt = getConversationFromUid(convUid_, *convModel_);
     if (convOpt.has_value()) {
-        lrc::api::conversation::Info& conversation = convOpt.value();
+        lrc::api::conversation::Info& conversation = *convOpt;
         cachedConv_ = &conversation;
     }
     return cachedConv_;