string a = null;
object b = null;
Button c = null;

var d = null; //das geht nicht
c = a; //geht offensichtlich auch nicht

The type of a null-literal is the null type (§11.2.7).

The null literal (§9.4.4.6) evaluates to the null value, which is used to denote a reference not pointing at any object or array, or the absence of a value. The null type has a single value, which is the null value. Hence an expression whose type is the null type can evaluate only to the null value. There is no way to explicitly write the null type and, therefore, no way to use it in a declared type.

Moreover, the null type can never be the type inferred for a type parameter (§25.6.4).

int? i = null;