Fix notNull annotation and fix test accordingly

What does this MR do (summary)?

Fixes a broken @NotNull annotation

What changes have you made (detailed)?

Fix import, fix test

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.