Text Box Problem

I am getting the following error in all of my text boxes:

The error seems to come at random when I am typing code in another part of the project. Once It starts I can’t seem to get rid of it. I have no idea why it is doing this. I can “fix the problem” by completed removing the unload script from the text-box (which causes other issues when moving between pages cos the text stays on the page).

Any idea what to do?