Filelist packageOf crashes when using assetExtensions

[jfo8000]

I am experimenting with extensions / resources / asset editors

I noticed that adding the following to pxtarget.json crashes

      "mathBlocks": true,
      "loopsBlocks": true,
      "logicBlocks": true,
      "variablesBlocks": true,
      "textBlocks": true,
      "functionBlocks": true,
      "listsBlocks": true ,
      "onStartUnDeletable": false,
      "assetExtensions": "foo" << I added this
      
    },
    

This is because assetExtensions create a new editor with a null ksPkg

It looks like half of the time the world copes when ksPkg is null, and the other half of the time it explodes.

From my sleuthing this seems to be the offender:

[jfo8000]

Repros 6.0.1, 6.9.11 pxt-core