fix: make is_roundtrip_possible return a boolean (Open-MSS#2985)

Author: TaniaRocks

mslib/msui/tableview.py

@@ -260,7 +260,7 @@ def is_roundtrip_possible(self):
             condition = first_waypoint.lat != last_waypoint.lat or first_waypoint.lon != last_waypoint.lon or \
                 first_waypoint.flightlevel != last_waypoint.flightlevel
 
-        return condition
+        return bool(condition)
 
     def update_roundtrip_enabled(self):
         self.btRoundtrip.setEnabled(self.is_roundtrip_possible())