User can backspace last character of variable and edit locked text

[DianaLease]

Describe the bug
A user can edit locked text around a variable by removing the last character of the variable and continuing to type. If a user clicks somewhere else, they can no longer edit the new text added to the locked text and the clause will be broken with no way to fix it.

To Reproduce
Steps to reproduce the behavior:

1. Add a clause while lockText is true
2. Remove the last character of a variable by using the backspace key
3. Continue to type after hitting backspace
4. See your text be added to the locked text area instead of inside of the highlighted variable

Expected behavior
A user should not be able to add text to the locked text area of a clause. If the user removes the last character of a variable and continues to type, they should still be inside of the variable.