ringtone: check for null account

Change-Id: I45226bfac353e1d0c76a66519473d1e420678730
diff --git a/ring-android/app/src/main/java/cx/ring/client/RingtoneActivity.java b/ring-android/app/src/main/java/cx/ring/client/RingtoneActivity.java
index a795162..00ee3da 100644
--- a/ring-android/app/src/main/java/cx/ring/client/RingtoneActivity.java
+++ b/ring-android/app/src/main/java/cx/ring/client/RingtoneActivity.java
@@ -71,7 +71,7 @@
     private Account mAccount;
     private TextView customRingtone;
     private ImageView customPlaying, customSelected;
-    private MediaPlayer mediaPlayer = new MediaPlayer();
+    private final MediaPlayer mediaPlayer = new MediaPlayer();
     private Disposable disposable;
 
     @Inject
@@ -87,6 +87,10 @@
         setContentView(R.layout.activity_ringtone);
         super.onCreate(savedInstanceState);
         mAccount = mAccountService.getAccount(getIntent().getExtras().getString(AccountEditionFragment.ACCOUNT_ID_KEY));
+        if (mAccount == null) {
+            finish();
+            return;
+        }
 
         /*Toolbar toolbar = findViewById(R.id.ringtoneToolbar);
         toolbar.setNavigationOnClickListener(view -> finish());*/