is_interstitial_loaded() always return false

[Nguyen Quang Huy]

**Godot version:**3.5.2

**AdMob Plugin version:**6.2.0

Issue description:
Method is_interstitial_loaded() always return false even I call load_interstitial() in the _ready() of main scene
I have a main scene, which has an child node AdMob. In _ready() I call load_interstitial() and try to call is_interstitial_loaded() when game over, to show ads, but it always return false.

Pls help me out.