show warning in preferences if currencies could not be retrieved from exchange rate source

src/main/java/com/sparrowwallet/sparrow/preferences/GeneralPreferencesController.java

@@ -18,6 +18,7 @@ import javafx.collections.FXCollections;
 import javafx.collections.ObservableList;
 import javafx.fxml.FXML;
 import javafx.scene.control.ComboBox;
+import javafx.scene.control.Label;
 import javafx.util.StringConverter;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -45,6 +46,9 @@ public class GeneralPreferencesController extends PreferencesDetailController {
     @FXML
     private ComboBox<ExchangeSource> exchangeSource;
 
+    @FXML
+    private Label currenciesLoadWarning;
+
     @FXML
     private UnlabeledToggleSwitch loadRecentWallets;
 
@@ -87,6 +91,9 @@ public class GeneralPreferencesController extends PreferencesDetailController {
             EventManager.get().post(new FeeRatesSourceChangedEvent(newValue));
         });
 
+        currenciesLoadWarning.managedProperty().bind(currenciesLoadWarning.visibleProperty());
+        currenciesLoadWarning.setVisible(false);
+
         blockExplorers.setItems(getBlockExplorerList());
         blockExplorers.setConverter(new StringConverter<>() {
             @Override
@@ -237,6 +244,8 @@ public class GeneralPreferencesController extends PreferencesDetailController {
             fiatCurrency.setDisable(true);
         }
 
+        currenciesLoadWarning.setVisible(exchangeSource.getValue() != ExchangeSource.NONE && currencies.isEmpty());
+
         //Always fire event regardless of previous selection to update rates
         EventManager.get().post(new FiatCurrencySelectedEvent(exchangeSource.getValue(), fiatCurrency.getValue()));
 

src/main/resources/com/sparrowwallet/sparrow/general.css

@@ -320,3 +320,7 @@ CellView > .text-input.text-field {
     -fx-background-color: -fx-control-inner-background;
 }
 
+.field-warning {
+    -fx-text-fill: rgb(238, 210, 2);
+    -fx-padding: 0 0 0 12;
+}

src/main/resources/com/sparrowwallet/sparrow/preferences/general.fxml

@@ -15,6 +15,7 @@
 <?import com.sparrowwallet.sparrow.control.UnlabeledToggleSwitch?>
 <?import com.sparrowwallet.sparrow.control.HelpLabel?>
 <?import com.sparrowwallet.sparrow.net.FeeRatesSource?>
+<?import org.controlsfx.glyphfont.Glyph?>
 
 <GridPane hgap="10.0" vgap="10.0" stylesheets="@preferences.css, @../general.css" xmlns="http://javafx.com/javafx/10.0.2-internal" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.sparrowwallet.sparrow.preferences.GeneralPreferencesController">
     <padding>
@@ -60,6 +61,11 @@
                         </FXCollections>
                     </items>
                 </ComboBox>
+                <Label fx:id="currenciesLoadWarning" text="Error retrieving currencies">
+                    <graphic>
+                        <Glyph fontFamily="Font Awesome 5 Free Solid" fontSize="12" icon="EXCLAMATION_TRIANGLE" styleClass="field-warning" />
+                    </graphic>
+                </Label>
             </Field>
         </Fieldset>
         <Fieldset inputGrow="SOMETIMES" text="Wallet" styleClass="wideLabelFieldSet">