Script error of Follow up button

When clicking the Follow up button, sometimes users see a script error as by the bottom left of corner of the browser.

This occurs since your form assistant is disabled. Obviously, this is a contradiction.  Follow up pane should get loaded in form assistant section, which is not available.

Form Assistant and Follow up
So, when you design the form, you have to think Follow up functionality together with form assistant.

1) Option 1: If you want follow up, your form assistant should be enabled
For this, go to customizations > Open relevant entity > click Forms and Views > Open Form > Open Form prosperities > Click display tab.
Then enable the Form assistant by clicking the check box.

2) Option 2: If you want form assistant to be disabled, follow up button should be removed
How to remove the button using JavaScript is discussed here.

Decide this in terms of your useability expectations.