CATROID-1631 Fix SavePlotActionTest

[Kale2605]

Fix in test and code

https://catrobat.atlassian.net/browse/CATROID-1631?atlOrigin=eyJpIjoiYWZmYjk5NGU0MGNhNDhlYjg2ZmZkZjdjMDMwM2E0ODkiLCJwIjoiaiJ9

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Frajhamster]

Looks good!