index out of range error

[NicksPatties]

Describe the bug
When typing an incorrect character as the last character in an exercise, the program crashes.

To Reproduce
Steps to reproduce the behavior:

1. sweet
2. type the exercise
3. last character, press any incorrect character
4. see error

Expected behavior
I should not encounter a bug.

Sweet version
v0.2.1

Desktop (please complete the following information):

• OS: Pop_OS
• Version: 22.04 LTS