Added method buildCity.

src/ch/zhaw/catan/SiedlerGame.java

@@ -243,8 +243,10 @@ public class SiedlerGame {
             return false;
         }
         //2. Check if Settlement has already been built
-        Settlement atCurrentPosition = (Settlement) board.getCorner(position);
-
+        Settlement atCurrentPosition = board.getCorner(position);
+        if (atCurrentPosition == null){
+            return false;
+        }
         //3. Can player build a City.
         if(!allPlayers.get(activePlayer).buildCity()){
             return false;