Why any electric field doesn't exist outside the conductive spherical shell?

The conductive spherical shell(green one) and the conductive sphere are given(blue one).

The inner sphere is charged with $Q~$ and the outer shell is grounded and not charged.

$V_{text{a}}:=text{potential at the inner surface of the shell}$

$V_{text{b}}:=text{potential at the outer surface of the shell}$

$E_{r}:=text{electric field outside the shell}$

The description says that $V_{text{a}}=V_{text{b}}=E_{r}=0$

It is obvious that $V_{text{a}}=V_{text{b}}=0$ however the problem for me is to get $E_{r}=0$

For instance if we apply Gauss law and include the shell with the sphere then the below equation must be held.

$$text{sum of electric fields}=frac{Q}{epsilon_{0}}$$

What should I consider for next?