RungeKuttaUtils : A reference can not be null

RungeKuttaUtils: A reference can not be null

C++ std

A reference is required to be initialized to refer to a valid object or function: see reference initialization. Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” obtained by dereferencing a null pointer, which causes undefined behavior.