JNI problems in unity

It is likely that the Java object you are trying to access has gone out of scope, or has a return value other than void.

So, when you make the call to CallStatic, you should probably quality the return value as an AndroidJavaObject.

Take a look at the C# script below:

 using (AndroidJavaClass unityClass = new AndroidJavaClass("com.unity3d.player.UnityPlayer") )
 {
    using (AndroidJavaObject activity = unityClass.GetStatic<AndroidJavaObject>("currentActivity") )
    {    
       activity.Call("startSensorService");            
    }
 }

You've probably seen this type of code in plugin examples. In the 2nd using statement, it calls a static member function of UnityPlayer to get the current activity object. If I called unityClass.GetStatic("currentActivity") without the qualifier, I'd get an error saying "can't find method id".

Also, if for some reason the return value from the Java code were null, a different kind of error would happen, which is also pretty common, the "JNI: Init'd AndroidJavaObject with null ptr" error.

The best to to avoid that error is to go into the Java code and make sure that these functions return something, or if they can't create another method like "hasInstance" or some such that can make sure the value you are about to return is a valid one.