Fix broken check

SignalServiceKit/src/Messages/OWSMessageSender.m

@@ -356,7 +356,7 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         OWSAssertDebug([message.body lengthOfBytesUsingEncoding:NSUTF8StringEncoding] <= kOversizeTextMessageSizeThreshold);
     }
     
-    if (!message.isGroupChatMessage) {
+    if (!message.thread.isGroupThread) {
         [NSNotificationCenter.defaultCenter postNotificationName:NSNotification.calculatingPoW object:[[NSNumber alloc] initWithUnsignedLongLong:message.timestamp]]; // Not really true but better from a UI point of view
     }