Crash when scanning WiFi QR code

Fix two crashes when scanning a WiFi QR code.
The crashes are due to the asynchronous nature of the scan/decode
process, where the result is delivered after the activity has been
destroyed.

Bug: 223640678
Test: manual, Settings -> Network & internet -> Internet
      Press the 'Scan QR code' icon
      Start to scan a WiFi QR code
      Just before then scan/decode completes, press Back
Change-Id: Ic19ca08590ed76ca9be46eabe3e0b17419fb0b44

src/com/android/settings/wifi/dpp/WifiDppQrCodeScannerFragment.java

@@ -168,10 +168,18 @@ public class WifiDppQrCodeScannerFragment extends WifiDppQrCodeBaseFragment impl
                     break;
 
                 case MESSAGE_SCAN_ZXING_WIFI_FORMAT_SUCCESS:
+                    final Context context = getContext();
+                    if (context == null) {
+                        // Context may be null if the message is received after the Activity has
+                        // been destroyed
+                        Log.d(TAG, "Scan success but context is null");
+                        return;
+                    }
+
                     // We may get 2 WifiConfiguration if the QR code has no password in it,
                     // one for open network and one for enhanced open network.
                     final WifiManager wifiManager =
-                            getContext().getSystemService(WifiManager.class);
+                            context.getSystemService(WifiManager.class);
                     final WifiNetworkConfig qrCodeWifiNetworkConfig =
                             (WifiNetworkConfig)msg.obj;
                     final List<WifiConfiguration> qrCodeWifiConfigurations =