Don't execute the check if the given server is null

app/src/main/java/com/frostnerd/dnschanger/activities/MainActivity.java

@@ -761,7 +761,7 @@ public class MainActivity extends NavigationDrawerActivity implements RuleImport
                     dialog.show();
                     ((MainFragment)currentFragment()).checkDNSReachability(new MainFragment.DNSReachabilityCallback() {
                         @Override
-                        public void checkFinished(List<IPPortPair> unreachable, List<IPPortPair> reachable) {
+                        public void checkFinished(@NonNull List<IPPortPair> unreachable, List<IPPortPair> reachable) {
                             dialog.dismiss();
                             if(unreachable.size() == 0){
                                 MainActivity.this.runOnUiThread(new Runnable() {

app/src/main/java/com/frostnerd/dnschanger/fragments/MainFragment.java

@@ -376,7 +376,7 @@ public class MainFragment extends Fragment {
             dialog.show();
             checkDNSReachability(new DNSReachabilityCallback() {
                 @Override
-                public void checkFinished(List<IPPortPair> unreachable, List<IPPortPair> reachable) {
+                public void checkFinished(@NonNull List<IPPortPair> unreachable, @NonNull List<IPPortPair> reachable) {
                     dialog.dismiss();
                     if(unreachable.size() == 0){
                         ((MainActivity)requireContext()).runOnUiThread(new Runnable() {
@@ -391,7 +391,7 @@ public class MainFragment extends Fragment {
                         _text = _text.replace("[x]", unreachable.size() + reachable.size() + "");
                         _text = _text.replace("[y]", unreachable.size() + "");
                         boolean customPorts = PreferencesAccessor.areCustomPortsEnabled(requireContext());
-                        for(IPPortPair p: unreachable)builder.append("- ").append(p.formatForTextfield(customPorts)).append("\n");
+                        for(IPPortPair p: unreachable) builder.append("- ").append(p.formatForTextfield(customPorts)).append("\n");
                         _text = _text.replace("[servers]", builder.toString());
                         final String text = _text;
                         ((MainActivity)requireContext()).runOnUiThread(new Runnable() {
@@ -490,13 +490,14 @@ public class MainFragment extends Fragment {
     }
 
     public static abstract class DNSReachabilityCallback{
-        private final List<IPPortPair> unreachable = new ArrayList<>();
-        private final List<IPPortPair> reachable = new ArrayList<>();
+        @NonNull private final List<IPPortPair> unreachable = new ArrayList<>();
+        @NonNull private final List<IPPortPair> reachable = new ArrayList<>();
         private int servers;
 
-        public abstract void checkFinished(List<IPPortPair> unreachable, List<IPPortPair> reachable);
+        public abstract void checkFinished(@NonNull List<IPPortPair> unreachable, @NonNull List<IPPortPair> reachable);
 
         public final void checkProgress(IPPortPair server, boolean reachable){
+            if(server == null)return;
             if(!reachable)unreachable.add(server);
             else this.reachable.add(server);
             if(this.unreachable.size() + this.reachable.size() >= servers)checkFinished(unreachable, this.reachable);

app/src/main/java/com/frostnerd/dnschanger/util/DNSQueryUtil.java

@@ -43,6 +43,7 @@ public class DNSQueryUtil {
     }
 
     public static void startDNSServerConnectivityCheck(@NonNull final IPPortPair server, @NonNull final Util.ConnectivityCheckCallback callback){
+        if(server == null)return;
         runAsyncDNSQuery(server, "frostnerd.com", false, Type.A, DClass.ANY, new Util.DNSQueryResultListener() {
             @Override
             public void onSuccess(Record[] response) {
@@ -58,6 +59,7 @@ public class DNSQueryUtil {
 
     public static void runAsyncDNSQuery(final IPPortPair server, final String query, final boolean tcp, final int type,
                                         final int dClass, final Util.DNSQueryResultListener resultListener, final int timeout){
+        if(server == null)return;
         new Thread(){
             @Override
             public void run() {
@@ -80,6 +82,7 @@ public class DNSQueryUtil {
 
     public static Record[] runSyncDNSQuery(final IPPortPair server, final String query, final boolean tcp, final int type,
                                           final int dClass, final int timeout){
+        if(server == null) return null;
         try {
             Resolver resolver = new SimpleResolver(server.getAddress());
             resolver.setPort(server.getPort());