fix potential crash (#5305)

I dont think this can even happen without some plugins or mods but its better to be safe than sorry.

core/src/mindustry/world/blocks/distribution/MassDriver.java

@@ -319,6 +319,7 @@ public class MassDriver extends Block{
 
         @Override
         public Point2 config(){
+            if(tile == null) return null;
             return Point2.unpack(link).sub(tile.x, tile.y);
         }