Fixed a crash with disabling river gen

src/main/java/biomesoplenty/common/world/layer/GenLayerRiverMixBOP.java

@@ -75,6 +75,9 @@ public class GenLayerRiverMixBOP extends BOPGenLayer
     }
     
     private boolean biomeSupportsRivers(int biomeId)
+    {
+        //Check if the biome id is valid
+        if (biomeId >= 0 && biomeId <= BiomeGenBase.getBiomeGenArray().length)
         {
             BiomeGenBase biome = BiomeGenBase.getBiome(biomeId);
 
@@ -83,6 +86,7 @@ public class GenLayerRiverMixBOP extends BOPGenLayer
                 BOPBiome bopBiome = (BOPBiome)biome;
                 return bopBiome.canGenerateRivers;
             }
+        }
 
         return true;
     }