Possible fix for lua crash #5269

Check if this instance has been disposed and don't call 'tick'
if it has.

Also remove the finalizer that was broken and wrong anyway.
'runtime' would never become null because it's readonly and
managed resources are freed automatically or may have already
been freed by then.

OpenRA.Game/Scripting/ScriptContext.cs

@@ -81,11 +81,12 @@ namespace OpenRA.Scripting
 		public ScriptGlobalAttribute(string name) { Name = name; }
 	}
 
-	public class ScriptContext
+	public class ScriptContext : IDisposable
 	{
 		public World World { get; private set; }
 		public WorldRenderer WorldRenderer { get; private set; }
 
+		bool disposed;
 		readonly MemoryConstrainedLuaRuntime runtime;
 		readonly LuaFunction tick;
 
@@ -188,26 +189,34 @@ namespace OpenRA.Scripting
 
 		public void Tick(Actor self)
 		{
-			if (error)
+			if (error || disposed)
 				return;
 
 			using (new PerfSample("tick_lua"))
 				tick.Call().Dispose();
 		}
 
-		public void Dispose()
+		protected void Dispose(bool disposing)
 		{
-			if (runtime == null)
+			if (disposed)
 				return;
 
+			if (disposing)
+				runtime.Dispose();
+
+			disposed = true;
+		}
+
+		public void Dispose()
+		{
+			Dispose(true);
 			GC.SuppressFinalize(this);
-			runtime.Dispose();
 		}
 
 		~ScriptContext()
 		{
-			if (runtime != null)
-				Game.RunAfterTick(Dispose);
+			// Dispose unmanaged resources only
+			Dispose(false);
 		}
 
 		static Type[] ExtractRequiredTypes(Type t)