Fields and Text block throws JS error when clicking Toolset buttons too soon

Symptoms

A Fields and Text block includes the Toolset buttons (such as the “Fields and Views” button under a Toolset icon, and Forms and conditional buttons). If you insert a Fields and Text block and then try to click on one of the Toolset buttons it may trigger a JS error:

Uncaught TypeError: can't access property "execCommand", i is null  ... from ... blocks.js:17:150997

Workaround

The problem occurs if you do not first click into the text box of the block, before then trying to use the buttons.

To avoid the issue, insert the Fields and Text block and then click into the text box where you intend to insert a field or other content from the buttons, and only then click the relevant button.