fix NullPointerException when trying to access config classes

src/main/java/uk/co/angrybee/joe/DiscordWhitelister.java

@@ -268,6 +268,12 @@ public class DiscordWhitelister extends JavaPlugin
 
     public static void ConfigSetup()
     {
+        mainConfig = new MainConfig();
+        customPrefixConfig = new CustomPrefixConfig();
+        customMessagesConfig = new CustomMessagesConfig();
+        permissionsConfig = new PermissionsConfig();
+        onWhitelistCommandsConfig = new OnWhitelistCommandsConfig();
+
         // Run this first, as it creates the root folder if it does not exist
         mainConfig.ConfigSetup();
         customPrefixConfig.ConfigSetup();

src/main/java/uk/co/angrybee/joe/configs/MainConfig.java

@@ -11,7 +11,7 @@ import java.util.Collections;
 
 // discord-whitelister.yml
 public class MainConfig extends Config {
-    MainConfig() {
+    public MainConfig() {
         fileName = "discord-whitelister.yml";
         file = new File(DiscordWhitelister.getPlugin().getDataFolder(), fileName);
         fileConfiguration = new YamlConfiguration();

src/main/java/uk/co/angrybee/joe/configs/OnWhitelistCommandsConfig.java

@@ -8,7 +8,7 @@ import java.io.IOException;
 import java.util.Arrays;
 
 public class OnWhitelistCommandsConfig extends Config {
-    OnWhitelistCommandsConfig() {
+    public OnWhitelistCommandsConfig() {
         fileName = "on-whitelist-commands.yml";
         file = new File(DiscordWhitelister.getPlugin().getDataFolder(), fileName);
         fileConfiguration = new YamlConfiguration();