fix potential crash. Fixes #2041

src/main/java/carpet/CarpetServer.java

@@ -40,6 +40,8 @@ import net.minecraft.server.MinecraftServer;
 import net.minecraft.server.commands.PerfCommand;
 import net.minecraft.server.level.ServerPlayer;
 
+import javax.annotation.Nullable;
+
 public class CarpetServer // static for now - easier to handle all around the code, its one anyways
 {
     public static MinecraftServer minecraft_server;
@@ -172,7 +174,7 @@ public class CarpetServer // static for now - easier to handle all around the co
         scriptServer = null;
     }
 
-    public static void onServerClosed(MinecraftServer server)
+    public static void onServerClosed(@Nullable MinecraftServer server)
     {
         // this for whatever reason gets called multiple times even when joining on SP
         // so we allow to pass multiple times gating it only on existing server ref
@@ -180,7 +182,7 @@ public class CarpetServer // static for now - easier to handle all around the co
         {
             if (scriptServer != null) scriptServer.onClose();
             // this is a mess, will cleanip onlly when global reference is gone
-            if (!Vanilla.MinecraftServer_getScriptServer(server).stopAll) {
+            if (server != null && !Vanilla.MinecraftServer_getScriptServer(server).stopAll) {
                 Vanilla.MinecraftServer_getScriptServer(server).onClose();
             }