Fix crash

SignalServiceKit/src/Loki/API/Public Chat/LokiPublicChatPoller.swift

@@ -155,10 +155,12 @@ public final class LokiPublicChatPoller : NSObject {
                 guard let messageID = message.uniqueId else { return print("[Loki] Failed to save public chat message.") }
                 storage.setIDForMessageWithServerID(UInt(messageServerID), to: messageID, in: transaction)
             }
+            DispatchQueue.main.async {
                 if let linkPreviewURL = OWSLinkPreview.previewUrl(forMessageBodyText: message.body, selectedRange: nil) {
                     message.generateLinkPreviewIfNeeded(fromURL: linkPreviewURL)
                 }
             }
+        }
         // Poll
         let _ = LokiPublicChatAPI.getMessages(for: publicChat.channel, on: publicChat.server).done(on: DispatchQueue.global()) { messages in
             func proceed() {