Add null check for 6 hour waiting time

Add null check for sixHourWaitingTime instant. This value can be null in
case plugin was disabled before user logged in and so sixHourWaitingTime
was never set.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

runelite-client/src/main/java/net/runelite/client/plugins/idlenotifier/IdleNotifierPlugin.java

@@ -409,6 +409,11 @@ public class IdleNotifierPlugin extends Plugin
 
 	private boolean check6hrLogout()
 	{
+		if (sixHourWarningTime == null)
+		{
+			return false;
+		}
+
 		if (Instant.now().compareTo(sixHourWarningTime) >= 0)
 		{
 			if (notify6HourLogout)