fix(mechanics): Don't attempt to land when the flagship is destroyed (e…

…ndless-sky#10739)

source/Engine.cpp

@@ -2047,7 +2047,7 @@ void Engine::HandleMouseClicks()
 						if(!planet->CanLand(*flagship))
 							Messages::Add("The authorities on " + planet->Name()
 									+ " refuse to let you land.", Messages::Importance::Highest);
-						else
+						else if(!flagship->IsDestroyed())
 						{
 							activeCommands |= Command::LAND;
 							Messages::Add("Landing on " + planet->Name() + ".", Messages::Importance::High);