You are listening to the wrong signal. You probably should use "TeX Jax Ready" instead. The TeX input jax is not loaded as an extension, so probably isn't available when the "End Extensions" signal is triggered.
Note that your solution doesn't prevent anyone from using `\require{HTML}` to load the HTML extension at runtime anyway. So you might want to remove the \require macro as well. (Issues like this is one reason to use the Safe extension, since we have covered more of the bases, though you are free to handle it however you feel best.)
Davide