ArgumentNullException: Value cannot be null when running a basic scene using the Unity plugin

[ih]

Hi,

I'm a hobbyist and I'm trying to use MuJoCo for learning more about deep reinforcement learning and robotics.

I'm looking for some help with getting started and I'm trying to follow this YouTube video creating a sphere that rolls down a plane.

I believe I've correctly installed the plugin following the docs and after following the video to create a plane and sphere when I hit run I get the exception

System.ArgumentNullException: Value cannot be null.
Parameter name: native
  at (wrapper managed-to-native) System.Runtime.InteropServices.Marshal.copy_to_unmanaged_fixed(System.Array,int,intptr,int,void*)
  at System.Runtime.InteropServices.Marshal.Copy (System.Byte[] source, System.Int32 startIndex, System.IntPtr destination, System.Int32 length) [0x0001e] in <b89873cb176e44a995a4781c7487d410>:0 
  at Mujoco.MjVfs.AddFile (System.String filename, System.String contents) [0x00072] in C:\Projects\mujoco\unity\Runtime\Tools\MjVfs.cs:60 
  at Mujoco.MjEngineTool.LoadModelFromString (System.String contents) [0x0000e] in C:\Projects\mujoco\unity\Runtime\Tools\MjEngineTool.cs:68 
  at Mujoco.MjScene.CompileScene (System.Xml.XmlDocument mjcf, System.Collections.Generic.IEnumerable`1[T] components) [0x00001] in C:\Projects\mujoco\unity\Runtime\Components\MjScene.cs:179 
  at Mujoco.MjScene.CreateScene (System.Boolean skipCompile) [0x00143] in C:\Projects\mujoco\unity\Runtime\Components\MjScene.cs:171 
  at Mujoco.MjScene.Start () [0x00008] in C:\Projects\mujoco\unity\Runtime\Components\MjScene.cs:89 

I am using Unity 2022.3.7f1 and MuJoCo 2.3.7 with Windows 11 Pro (AMD Ryzen 7 3700X).

I'm familiar with Unity, but have never used MuJoCo.

Here is a model I exported of the scene:

minimal XML

```XML ```

Here are some screenshots:

I inspected the arguments to Marshal.Copy and none of them were null.

I'm not sure if this is a bug or if I'm missing a setup step or something like that. Thanks for any help!

[Balint-H]

Could you expand the Sphere Body GameObject in the hierarchy to see what components were added? You also might be missing a MjGlobalOptions component from your scene.

[Balint-H]

You can also export the Unity scene to XML to see what is used to run the Mujoco scene

[Balint-H]

Could you try running it with the 2.3.7 release or earlier version of the plugin? Lastly, this error may sometimes be caused by a mismatch in DLL and plugin version.

[ih]

Thanks for your suggestions and sorry for the delayed reply, I was traveling recently.

I tried all the above and what worked was reverting the MuJoCo repo to the commit tagged 2.3.6.

Seems like maybe there is a bug, but I don't see an option to change the label on this issue. Let me know if it'd be helpful to create a new issue/close this one.

[brownbrowny]

I get the same error message when using version 2.3.6. for a robot model when pressing play. Thus I switched back to version 2.3.5.

[anayebi]

I also get this error with the latest version of the Mujoco github repo, but downgrading to the release version of 2.3.7 works for me.