alias this and null reference

[Namespace]

By the following code i get a normal Access Violation.
My question is: why? Even if "f0" is null, the object must be
converted to Ref and there i check if the given object is null.

Here is the Code:
[code]
// Ref.d

struct Ref(T : Object) {
private:
	T _val;

public:
	@disable
	this();

	@disable
	this(typeof(null));

	@disable
	typeof(this) opAssign(typeof(null));

	this(T val) {
		assert(val !is null);

		this._val = val;
	}

	@property
	T get() {
		return this._val;
	}
}

mixin template TRef(T) {
	@property
	Ref!(T) getRef() {
		assert(this !is null);

		return Ref!(T)(this);
	}

	alias getRef this;
}

// main.d
class Foo {
private:
	string _val;

public:
	this() { }

	void Set(string val) {
		this._val = val;
	}

	void speak() const {
		writeln(this._val);
	}

	mixin TRef!(typeof(this));
}

void with_ref(Ref!(Foo) rf) {
	rf.get.speak();
}

void without_ref(Foo f) {
	f.speak();
}

void main() {
	Foo f0; // f0 is null

	with_ref(f0); // converting with alias this. But what if f0 is
null?

	Foo f1 = new Foo();
	f1.Set("Foo!");

	with_ref(f1);
	without_ref(f1);

	readln();
}
[/code]