Application runs perfectly in Visual Studio, yet when the executable runs it throws NullReferenceException. File reading issue?

I have written a C# application in Visual Studio (using Avalonia, irrelevant for this question).

c#: Mac application runs perfectly in visual studio, yet when the executable runs it throws NullReferenceException. File reading issue?

I have written a c# application in Visual Studio (using Avalonia, irrelevant for this question).