Fix Bug in .NET Core Web API Endpoint

I have a small ASP.NET Core Web API (.NET 8) project with a bug in one of the existing endpoints. The endpoint is supposed to return a filtered list of records from a SQL Server database using Entity Framework Core, but it's currently throwing a NullReferenceException under certain conditions. I need someone to identify the root cause, fix it, and add basic null-handling so it doesn't happen again. Scope of Work: Review the provided controller and service class (~150 lines of code) Identify and fix the null reference bug Add appropriate null checks / guard clauses Write 1–2 simple unit tests (xUnit) confirming the fix Briefly document the root cause and fix in a short comment or README note Requirements: Solid experience with C# and ASP.NET Core (6/7/8) Familiarity with Entity Framework Core Basic understanding of xUnit or similar testing framework Ability to turn this around within 24–48 hours Deliverables: Updated code (via Git patch, zip, or pull request) Passing unit tests Short explanation of the bug and fix Additional Notes: Source code and a sample dataset will be provided upon acceptance Open to a quick call if clarification is needed This is a trial task — future work available for the right candidate