Fix nullable bug

[bakatsuyuki]

There is a bug with null safety like this:

/Users/bakatsuyuki/flutter/.pub-cache/hosted/pub.dartlang.org/arcore_flutter_plugin-0.1.0-null-safety.3/android/src/main/kotlin/com/difrancescogianmarco/arcore_flutter_plugin/ArcoreFlutterPlugin.kt: (32, 74): Type mismatch: inferred type is Activity? but Activity was expected

FAILURE: Build failed with an exception.

So I fixed it to return if the activity is null.

[amitgeed]

Giving Exception
[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: PlatformException(error, null cannot be cast to non-null type android.view.View, null, java.lang.NullPointerException: null cannot be cast to non-null type android.view.View