Fix #3881 by checking for result being null

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

src/main/java/com/owncloud/android/authentication/AuthenticatorActivity.java

@@ -2415,7 +2415,7 @@ public class AuthenticatorActivity extends AccountAuthenticatorActivity
 
             String result = data.getStringExtra("com.blikoon.qrcodescanner.got_qr_scan_relult");
 
-            if (!result.startsWith(getString(R.string.login_data_own_scheme))) {
+            if (result == null || !result.startsWith(getString(R.string.login_data_own_scheme))) {
                 mServerStatusIcon = R.drawable.ic_alert;
                 mServerStatusText = "QR Code could not be read!";
                 showServerStatus();