Call to "Input" into a static class => null ref

I try to make my own InputManager (a static class) but when i do a call to the Unity Input class into a method (Input.GetMouseButtonDown() for example), i got a NullReferenceException once…
And it works properly but i still have a error.

The best way to make managers in unity would be using Singleton pattern, it would be much better than all static classes.