Fixed an error occurring when adding a player to the whitelist (#53)

Author: Dev7ex

multiworld-bukkit/src/main/java/com/dev7ex/multiworld/world/DefaultWorldManager.java

@@ -31,10 +31,7 @@
 import java.io.File;
 import java.io.IOException;
 import java.text.SimpleDateFormat;
-import java.util.Arrays;
-import java.util.Calendar;
-import java.util.Collections;
-import java.util.Objects;
+import java.util.*;
 import java.util.logging.Level;
 
 /**
@@ -215,7 +212,7 @@ public void createWorld(@NotNull final String creatorName, @NotNull final String
                 .setEndWorldName(defaultProperties.getString(WorldDefaultProperty.END_WORLD))
                 .setNetherWorldName(defaultProperties.getString(WorldDefaultProperty.NETHER_WORLD))
                 .setNormalWorldName(defaultProperties.getString(WorldDefaultProperty.NORMAL_WORLD))
-                .setWhitelist(Collections.emptyList())
+                .setWhitelist(new ArrayList<>())
                 .setWhitelistEnabled(defaultProperties.getBoolean(WorldDefaultProperty.WHITELIST_ENABLED))
                 .build();
 
@@ -279,7 +276,7 @@ public void createWorld(@NotNull final String creatorName, @NotNull final String
                 .setEndWorldName(defaultProperties.getString(WorldDefaultProperty.END_WORLD))
                 .setNetherWorldName(defaultProperties.getString(WorldDefaultProperty.NETHER_WORLD))
                 .setNormalWorldName(defaultProperties.getString(WorldDefaultProperty.NORMAL_WORLD))
-                .setWhitelist(Collections.emptyList())
+                .setWhitelist(new ArrayList<>())
                 .setWhitelistEnabled(defaultProperties.getBoolean(WorldDefaultProperty.WHITELIST_ENABLED))
                 .build();
 
@@ -361,7 +358,7 @@ public void createWorld(@NotNull final String creatorName, @NotNull final String
                 .setEndWorldName(defaultProperties.getString(WorldDefaultProperty.END_WORLD))
                 .setNetherWorldName(defaultProperties.getString(WorldDefaultProperty.NETHER_WORLD))
                 .setNormalWorldName(defaultProperties.getString(WorldDefaultProperty.NORMAL_WORLD))
-                .setWhitelist(Collections.emptyList())
+                .setWhitelist(new ArrayList<>())
                 .setWhitelistEnabled(defaultProperties.getBoolean(WorldDefaultProperty.WHITELIST_ENABLED))
                 .build();