message: Report an error when Android Keystore returns a null key.
diff: @@ -21,6 +21,7 @@ import android.os.Build.VERSION_CODES;
  import com.google.crypto.tink.Aead;
  import java.io.IOException;
  import java.security.GeneralSecurityException;
  +import java.security.InvalidKeyException;
  import java.security.KeyStore;
  import javax.crypto.Cipher;
  import javax.crypto.SecretKey;
  @@ -46,6 +47,9 @@ public final class AndroidKeystoreAesGcm implements Aead {
  KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
  keyStore.load(null /* param */);
  key = (SecretKey) keyStore.getKey(keyId, null /* password */);
  + if (key == null) {
  + throw new InvalidKeyException("Keystore cannot load the key with ID: " + keyId);
  + }
  }
  @Override
  