questlist: Ensure filter has non-null state on startup

Prior to this commit, the filter state was only set initially when
logging in, causing NPEs and errant plugin behavior if toggled on when
already logged in.

runelite-client/src/main/java/net/runelite/client/plugins/questlist/QuestListPlugin.java

@@ -96,12 +96,14 @@ public class QuestListPlugin extends Plugin
 	@Override
 	protected void startUp()
 	{
+		currentFilterState = QuestState.ALL;
 		clientThread.invoke(this::addQuestButtons);
 	}
 
 	@Override
 	protected void shutDown()
 	{
+		currentFilterState = null;
 		Widget header = client.getWidget(WidgetInfo.QUESTLIST_BOX);
 		if (header != null)
 		{