Added url validity check

mobile/src/main/java/org/openhab/habdroid/ui/PreferencesActivity.java

@@ -26,6 +26,7 @@ import android.text.TextUtils;
 import android.util.Log;
 import android.view.MenuItem;
 import android.widget.EditText;
+import android.widget.Toast;
 
 import androidx.annotation.DrawableRes;
 import androidx.annotation.Nullable;
@@ -44,6 +45,8 @@ import org.openhab.habdroid.util.CacheManager;
 import org.openhab.habdroid.util.Constants;
 import org.openhab.habdroid.util.Util;
 
+import java.net.MalformedURLException;
+import java.net.URL;
 import java.util.BitSet;
 
 import static org.openhab.habdroid.util.Util.getHostFromUrl;
@@ -248,11 +251,14 @@ public class PreferencesActivity extends AppCompatActivity {
                         R.string.settings_current_default_sitemap, currentDefaultSitemapLabel));
             }
             openLichtUrl.setOnPreferenceChangeListener((preference, newValue) -> {
-                if (!newValue.toString().isEmpty()) {
+                try {
+                    new URL(newValue.toString()).toURI();
                     preference.setSummary(newValue.toString());
                     return true;
-                }
+                } catch (Exception e) {
+                    Toast.makeText(this.getActivity(), "URL not valid. Don't forget to add http(s):// at the beginning!", Toast.LENGTH_LONG).show();
                     return false;
+                }
             });
             updateOpenLichtUrlPreference(openLichtUrl);
             updateConnectionSummary(Constants.SUBSCREEN_LOCAL_CONNECTION,