[Functionality Bug] Unexpected Behaviour of Mark Command

[Lysire]

Describe the bug
The command mark 1 a/present does not behave as expected when executing the steps below.

To Reproduce
Steps to reproduce the behavior:

1. Create a new session (I typed in add-session s/CS2100 dt/03-10-2002 1800)
2. Execute lowest-score command
3. Click on the session to go back
4. Execute mark 1 a/present
5. See error

Expected behavior
The mark 1 a/present command should cause student 1's attendance to be reflected as "present"

Screenshots

Desktop (please complete the following information):

• OS: Windows 10
• Browser: N/A
• Version: N/A

Smartphone (please complete the following information):
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Additional context
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