fix potential NPE

src/main/java/com/owncloud/android/jobs/AccountRemovalJob.java

@@ -73,7 +73,10 @@ public class AccountRemovalJob extends Job implements AccountManagerCallback<Boo
             // disable contact backup job
             ContactsPreferenceActivity.cancelContactBackupJobForAccount(context, account);
 
-            am.removeAccount(account, this, null);
+
+            if (am != null) {
+                am.removeAccount(account, this, null);
+            }
 
             FileDataStorageManager storageManager = new FileDataStorageManager(account, context.getContentResolver());