fix(chatform): fix a crash when there are no messages to load

src/widget/form/genericchatform.cpp

@@ -1099,6 +1099,10 @@ void GenericChatForm::loadHistoryLower()
 
 void GenericChatForm::loadHistoryUpper()
 {
+    if (messages.empty()) {
+        return;
+    }
+
     auto msg = messages.crbegin()->second;
     loadHistoryFrom(QDateTime());
     chatWidget->scrollToLine(msg);