Fix FlowBench argument parsing and icon resource access

Summary:
See T29195: Fix FlowBench argument parsing and icon resource access.

Fixes T29195.

Test Plan: Tested manually.

Reviewers: floca, O3 MITK Reviewer Group II, O1 MITK Reviewer Group I

Reviewed By: floca, O3 MITK Reviewer Group II, O1 MITK Reviewer Group I

Maniphest Tasks: T29195

Differential Revision: https://phabricator.mitk.org/D660