ALTER PAGE cannot create or populate an empty classic-grid control bar; DROP of the last button deletes the bar itself

[MohamedGamalEzzatAli]

ALTER PAGE cannot create or populate an empty classic-grid control bar — and DROP of the last button deletes the bar, making the state unrecoverable via MDL

Summary

A classic data grid's control bar is reachable by MDL only through its EXISTING buttons
(INSERT BEFORE/AFTER <barButton> and SET ... ON <barButton> both work — nice!). But there
is no way to bootstrap one: the bar itself is not addressable, the grid refuses INSERT INTO,
and DROP WIDGET of the bar's last button silently removes the bar element too — so one
scripted cleanup pass later, the grid's toolbar is permanently out of MDL's reach and only
Studio Pro can bring it back.

Since CREATE PAGE's DATAGRID authors a DataGrid2 (pluggable), classic grids only exist as
Studio-made content — which makes their bars exactly the thing migration/refactor scripts need
to edit, and exactly the thing that currently dead-ends.

Reproduction (v0.19.0, on a fresh blank app — mx create-project ships the page used here)

The stock Administration.Account_Overview has a classic grid with a control bar:

datagrid dataGrid21 ( DataSource: database from Administration.Account ... ) {
  controlbar controlBar1 {
    actionbutton actionButton1 (Caption: 'New local user', ...)
    ...

1. Works today (for the record): SET (Caption = 'X') ON actionButton1 and
   INSERT AFTER actionButton1 { actionbutton btnNew (...) } both succeed.

2. Drop the bar's buttons:

ALTER PAGE Administration.Account_Overview {
  DROP WIDGET actionButton1;
  DROP WIDGET actionButton2;
};

Succeeds — and DESCRIBE afterwards shows no controlbar at all: the bar element was
removed together with its last child. (If that cascade is intended, it still deserves a
mention in the DROP output, since it deletes an element the statement never named.)

3. Every route back in is refused:

ALTER PAGE ... { INSERT INTO controlBar1 { actionbutton ... } };
Error: failed to insert: widget "controlBar1" not found. DataGrid2 columns are addressed by
a derived name (the bound attribute, or the caption), ...

(Also refused with this message BEFORE the drop, while the bar visibly existed with buttons —
the bar is not in the addressable-widget namespace at all, and the DataGrid2-columns hint is a
red herring for a classic grid.)

ALTER PAGE ... { INSERT INTO dataGrid21 { actionbutton ... } };
Error: failed to insert: cannot INSERT INTO "dataGrid21" (CustomWidgets$CustomWidget): it is
not a simple container — use INSERT BEFORE/AFTER a widget inside the target column or tab

Two oddities in that message worth a look while you're there: the classic grid is reported as
CustomWidgets$CustomWidget, and the suggested fix (anchor on a widget inside the target)
cannot apply — the grid's only insertable children live in the bar that no longer exists.

Suggested direction (any one closes the gap)

1. Make INSERT INTO <gridName> valid for grid-family widgets with actionbutton/controlbar
   children — auto-creating the control bar when absent (mirrors what Studio Pro's UI does on
   the first button drag).
2. Or make the bar addressable: INSERT INTO <controlBarName> { actionbutton ... }.
3. Or at minimum keep an emptied bar in the model (or disclose its deletion in the DROP
   output), so INSERT INTO has something to target once 1/2 exist.

Environment

mxcli v0.19.0 (b313ff0), Windows amd64, Mendix 11.12.1, MPRv2; observed identically on a
fresh mx create-project app (Administration module v4.3.2) and an existing app (v4.5.0).