fix: cleanly exit on shutdown (#5537)

Co-authored-by: Mm2PL <mm2pl+gh@kotmisia.pl>
Co-authored-by: Nerixyz <nerixdev@outlook.de>

src/common/Channel.cpp

@@ -26,7 +26,7 @@ namespace chatterino {
 // Channel
 //
 Channel::Channel(const QString &name, Type type)
-    : completionModel(*this, nullptr)
+    : completionModel(new TabCompletionModel(*this, nullptr))
     , lastDate_(QDate::currentDate())
     , name_(name)
     , messages_(getSettings()->scrollbackSplitLimit)