Fix ConnectivityManager.requestNetwork crash issue when try again

- When the user requests "try again", the Network-Request-Dialog will be
  recreated. If the Wi-Fi framework sends the callback result at this
  time, the Network-Request-Dialog may not be ready at the moment,
  causing the NPE to crash.

- Add NPE checks to avoid crash issues.

Bug: 220679437
Test: manual test
make RunSettingsRoboTests \
  ROBOTEST_FILTER=NetworkRequestDialogActivityTest

Change-Id: I8e48a29649e72d9f8c67c4ac24f0eb27ba600dae

src/com/android/settings/wifi/NetworkRequestDialogActivity.java

@@ -58,12 +58,17 @@ public class NetworkRequestDialogActivity extends FragmentActivity implements
     final static String EXTRA_IS_SPECIFIED_SSID =
         "com.android.settings.wifi.extra.REQUEST_IS_FOR_SINGLE_NETWORK";
 
-    @VisibleForTesting NetworkRequestDialogBaseFragment mDialogFragment;
+    @VisibleForTesting
+    NetworkRequestDialogBaseFragment mDialogFragment;
+    @VisibleForTesting
+    boolean mIsSpecifiedSsid;
+    @VisibleForTesting
+    boolean mShowingErrorDialog;
+    @VisibleForTesting
+    ProgressDialog mProgressDialog;
+
     private NetworkRequestUserSelectionCallback mUserSelectionCallback;
-    private boolean mIsSpecifiedSsid;
-    private boolean mShowingErrorDialog;
     private WifiConfiguration mMatchedConfig;
-    @VisibleForTesting ProgressDialog mProgressDialog;
 
     @Override
     protected void onCreate(@Nullable Bundle savedInstanceState) {
@@ -103,7 +108,8 @@ public class NetworkRequestDialogActivity extends FragmentActivity implements
         mDialogFragment.show(getSupportFragmentManager(), TAG);
     }
 
-    private void dismissDialogs() {
+    @VisibleForTesting
+    void dismissDialogs() {
         if (mDialogFragment != null) {
             mDialogFragment.dismiss();
             mDialogFragment = null;
@@ -174,7 +180,9 @@ public class NetworkRequestDialogActivity extends FragmentActivity implements
             return;
         }
 
-        mDialogFragment.onUserSelectionCallbackRegistration(userSelectionCallback);
+        if (mDialogFragment != null) {
+            mDialogFragment.onUserSelectionCallbackRegistration(userSelectionCallback);
+        }
     }
 
     @Override
@@ -201,7 +209,9 @@ public class NetworkRequestDialogActivity extends FragmentActivity implements
             return;
         }
 
-        mDialogFragment.onMatch(scanResults);
+        if (mDialogFragment != null) {
+            mDialogFragment.onMatch(scanResults);
+        }
     }
 
     @Override