IgxGridState is not working with IgxHierarchicalGrid

[bmtheo]

Description

When using the IgxGridState directive with IgxHierarchicalGrid it is throwing an error when trying to getState
"ERROR TypeError: Cannot read property 'columns' of null"

• igniteui-angular version:
  "igniteui-angular": "^9.0.6",
• browser:
  Tested on Chrome (Version 80.0.3987.149), Firefox (73.0.1) & Edge (Version 80.0.361.69)

Steps to reproduce

https://stackblitz.com/edit/igx-hierarchical-grid-persistence

1. Move a column around
2. Click on Save
3. The error is printed in the console and the state is not saved

Result

Error is printed and state is not usable

Expected result

No error then we can save/restore the state

Attachments

https://stackblitz.com/edit/igx-hierarchical-grid-persistence

Error in Chrome :

Error in Firefox :

The error in Stackblitz is not printed correctly, please consider reproducing in local to have the full error

Explanation of error

The error occurs in igniteui-angular-js line 59584 in function "getColumns()" (I don't have sourcemap) because this.grid is null.
I think this.grid is null because I'm using IgxHierarchicalGrid and not IgxGrid but I didn't find anything else to use in the doc/code (for example I looked for a IgxHierarchicalGridStateDirective)

I labeled this as a bug because the spec (https://github.com/IgniteUI/igniteui-angular/wiki/Grid-State-Persistence) specifies that it should work with all grids

If I'm mistaken and I just used the wrong directive, please show me the one to use and consider updating the doc

[ChronosSF]

@bmtheo , the IgxGridState directive was only implemented for state persistence of the igxGrid component, not for the Hierarchical or Tree grids. This should be considered a feature request.

[zdrawku]

Specification is updated

[radomirchev]

@bmtheo the development of the feature request is starting form the current sprint.