Javascript error trying to hide the "Show Rich-Text editor" button

Adding a multiplce choice question:

1. Fill the question text.
2. Fill the answer for the option A and set this option as the correct answer.
3. Fill the answer for the option C (leave the answer for the option B as empty).
4. Click on the "Show Rich-Text editor" button for the answer A.
5. Click on the "Save" button.

The javascript console shows the error:

"Uncaught TypeError: Cannot read property 'style' of null"

This is beacuse the line...

document.getElementById(client_id + "_toggle").style.display = "none";

in samigo-global.js is trying to hide the "Show Rich-Text editor" button when the button is already removed.