Null Pointer Error in the Code Base

[Sam2much96]

Godot version:
3.6
AdMob Plugin version:

Issue description:

I believe what causes the banner error and other random errors is a null pointer error caused from the null operator in the gdscript and in the java code. Ideally, the codebase should expect a String variable or a Node Type rather than a null variable that is set to a Node Object during run time. I believe modifying the source code and admob.gd to expect a Node Type and String Parameters of max 32 for the banner, interstitial and rewarded video variables would fix this problem.