App crashes when optional flag value not provided

[nus-pe-bot]

Steps to Replicate

1. Launch the App
2. Type the command filter t

Expected

Command is rejected with some error message

Actual

App crashes

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_{[original: nus-cs2113-AY2223S2/pe-interim#2900] [original labels: type.FunctionalityBug severity.Medium]}

[heejet]

Team's Response

No details provided by team.

Duplicate status (if any):

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