[Designer] The 'Add a Column' button does not work in the Adaptive Card Designer

Problem Description

Creating a card layout in the online designer (https://adaptivecards.microsoft.com/designer.html) adding a column set shows a button which previously allowed users to "Add a Column". Clicking this button in Chrome or Edge does not work, making it impossible to design a column layout in the online designer without manually editing the json.

Card JSON

{
    "type": "AdaptiveCard",
    "$schema": "https://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.6",
    "body": [
        {
            "type": "TextBlock",
            "text": "Example card",
            "wrap": true
        },
        {
            "type": "ColumnSet"
        }
    ]
}

Screenshot

Open the Adaptive Card Designer at https://adaptivecards.microsoft.com/designer.html and reproduce the issue with the provided ColumnSet card JSON in Chrome or Edge. Trace the Add a Column button from the ColumnSet editor; done means clicking it adds a column without manual JSON editing.