NullEngine dispose throws error

Hi,

when I dispose  null engine after unit test get error;

// before unit test
var engine = new NullEngine();
var scene = new Scene(engine);

// after unit test is executed
engine.dispose();

TypeError: Cannot read property 'deleteProgram' of undefined
      
      at NullEngine.Object.<anonymous>.Engine._deleteProgram (node_modules/babylonjs/dist/preview release/babylon.max.js:9898:26)
      at NullEngine.Object.<anonymous>.Engine.releaseEffects (node_modules/babylonjs/dist/preview release/babylon.max.js:11840:22)
      at NullEngine.Object.<anonymous>.Engine.dispose (node_modules/babylonjs/dist/preview release/babylon.max.js:11874:18)

Using: 3.1.1 babylon