No null in Collection/MapGameRule; resolves #993

api/src/main/java/de/erethon/dungeonsxl/api/dungeon/CollectionGameRule.java

@@ -72,7 +72,7 @@ public class CollectionGameRule<T, V extends Collection<T>> extends GameRule<V>
         Object value = config.get(getKey());
         if (reader != null) {
             V v = reader.read(api, value);
-            container.setState(this, v);
+            setStateWithoutNull(container, v);
             return v;
         }
 
@@ -82,10 +82,17 @@ public class CollectionGameRule<T, V extends Collection<T>> extends GameRule<V>
         } catch (ClassCastException exception) {
             return null;
         }
-        container.setState(this, v);
+        setStateWithoutNull(container, v);
         return v;
     }
 
+    private void setStateWithoutNull(GameRuleContainer container, V v) {
+        while (v != null && v.contains(null)) {
+            v.remove(null);
+        }
+        container.setState(this, v);
+    }
+
     @Override
     public void merge(GameRuleContainer overriding, GameRuleContainer subsidiary, GameRuleContainer writeTo) {
         V writeToState = writeTo.getState(this);

api/src/main/java/de/erethon/dungeonsxl/api/dungeon/MapGameRule.java

@@ -68,6 +68,7 @@ public class MapGameRule<TK, TV, V extends Map<TK, TV>> extends GameRule<V> {
         if (v == null) {
             return null;
         }
+        v.remove(null); // Do not allow null values
         container.setState(this, v);
         return v;
     }