Repository: godot
Issue: [Mono] 3.2.2 Android Plugin fails to call GodotLib.calldeferred with Mono Object
Labels: bug,platform:android,topic:porting,topic:dotnet

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**Godot version:**
<!-- Specify commit hash if using non-official build. -->
3.2.2.stable.mono.official

**OS/device including version:**
<!-- Specify GPU model, drivers, and the backend (GLES2, GLES3, Vulkan) if graphics-related. -->
Android with GLES3

**Issue description:**
<!-- What happened, and what was expected. -->
Using this plugin from community https://github.com/Shin-NiL/Godot-Android-Admob-Plugin
I tried to migrate the admob.gd to C#
from https://github.com/Shin-NiL/Godot-Android-Admob-Plugin/blob/main/demo/main.gd
to https://gist.github.com/ricardoalcantara/6dddf96f35f26d168a66757f8ab1109f

I achieve to init and call methods from C# but when the Java plugin tries to call GodotLib.calldeferred it fails with this error message

```
07-18 02:16:06.284 16880 16880 W godot   : AdMob: onAdLoaded
07-18 02:16:06.284 16880 16880 E godot   : **ERROR**: Condition "!obj" is true.
07-18 02:16:06.284 16880 16880 E godot   :    At: platform/android/java_godot_lib_jni.cpp:484:Java_org_godotengine_godot_GodotLib_calldeferred() - Condition "!obj" is true.
07-18 02:16:06.449 16880 16880 W godot   : AdMob: onRewardedVideoAdLoaded
07-18 02:16:06.449 16880 16880 E godot   : **ERROR**: Condition "!obj" is true.
07-18 02:16:06.449 16880 16880 E godot   :    At: platform/android/java_godot_lib_jni.cpp:484:Java_org_godotengine_godot_GodotLib_calldeferred() - Condition "!obj" is true.
07-18 02:16:06.490 16880 16880 W godot   : AdMob: onAdLoaded
07-18 02:16:06.490 16880 16880 E godot   : **ERROR**: Condition "!obj" is true.
07-18 02:16:06.490 16880 16880 E godot   :    At: platform/android/java_godot_lib_jni.cpp:484:Java_org_godotengine_godot_GodotLib_calldeferred() - Condition "!obj" is true.
```
this line fails https://github.com/Shin-NiL/Godot-Android-Admob-Plugin/blob/main/admob-plugin/godotadmob/src/main/java/shinnil/godot/plugin/android/godotadmob/Banner.java#L52

the get_instance_id() shows the same ID with GDScript, it's just cannot call back the Object from Java when it's C#

**Steps to reproduce:**
Download the demo.zip from the following section

![image](https://user-images.githubusercontent.com/2466466/87845814-4ddf6f80-c8a1-11ea-832a-e268e50b4661.png)
Install Android Build Template

![image](https://user-images.githubusercontent.com/2466466/87845828-723b4c00-c8a1-11ea-975f-46cd184382d1.png)
Add the sample ID in AndroidManifest.xml
```xml
<!-- Sample AdMob App ID: ca-app-pub-3940256099942544~3347511713 -->
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713"/>
```

Build and Run on Android Device

**Minimal reproduction project:**
<!-- A small Godot project which reproduces the issue. Drag and drop a zip archive to upload it. -->
[demo.zip](https://github.com/godotengine/godot/files/4941140/demo.zip)

** Workaround **
I Created a C# class that proxy the GDScript class.
https://gist.github.com/ricardoalcantara/c238cd07286b33d6ffbcecc87adb3684

> In my project both gd and c# were set as autoload (Singletons).