(core) Disable undo/redo in detached formula editor

Summary:
Undo and redo were causing errors to be thrown while the editor was detached. In the
interest of time, we'll disable undo/redo until we have a chance to look at whether
we can support it in the editor.

Test Plan: Manual.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3959

app/client/components/UndoStack.ts

@@ -25,7 +25,7 @@ export interface IUndoState {
  * position in this stack. Undo and redo actions are generated and sent to the server here.
  */
 export class UndoStack extends dispose.Disposable {
-
+  public isDisabled: Observable<boolean>;
   public undoDisabledObs: ko.Observable<boolean>;
   public redoDisabledObs: ko.Observable<boolean>;
   private _gristDoc: GristDoc;
@@ -40,6 +40,8 @@ export class UndoStack extends dispose.Disposable {
   public create(log: MinimalActionGroup[], options: {gristDoc: GristDoc}) {
     this._gristDoc = options.gristDoc;
 
+    this.isDisabled = Observable.create(this, false);
+
     // TODO: _stack and _linkMap grow without bound within a single session.
     // The top of the stack is stack.length - 1. The pointer points above the most
     // recently applied (not undone) action.
@@ -99,15 +101,27 @@ export class UndoStack extends dispose.Disposable {
   }
 
   // Send an undo action. This should be called when the user presses 'undo'.
-  public sendUndoAction(): Promise<void> {
+  public async sendUndoAction(): Promise<void> {
+    if (this.isDisabled.get()) { return; }
+
     return this._undoChain.add(() => this._sendAction(true));
   }
 
   // Send a redo action. This should be called when the user presses 'redo'.
-  public sendRedoAction(): Promise<void> {
+  public async sendRedoAction(): Promise<void> {
+    if (this.isDisabled.get()) { return; }
+
     return this._undoChain.add(() => this._sendAction(false));
   }
 
+  public enable(): void {
+    this.isDisabled.set(false);
+  }
+
+  public disable(): void {
+    this.isDisabled.set(true);
+  }
+
   private async _sendAction(isUndo: boolean): Promise<void> {
     // Pick the action group to undo or redo.
     const ag = this._stack[isUndo ? this._pointer - 1 : this._pointer];