Introduce CoinMarketCap stub to prevent error in existing clients

pricenode/src/main/java/bisq/price/spot/providers/CoinMarketCap.java

@@ -0,0 +1,53 @@
+/*
+ * This file is part of Bisq.
+ *
+ * Bisq is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Bisq is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Bisq. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package bisq.price.spot.providers;
+
+import bisq.price.spot.ExchangeRate;
+import bisq.price.spot.ExchangeRateProvider;
+
+import org.springframework.core.annotation.Order;
+import org.springframework.stereotype.Component;
+
+import java.time.Duration;
+
+import java.util.Collections;
+import java.util.Set;
+
+/**
+ * Stub implementation of CoinMarketCap price provider to prevent NullPointerExceptions within legacy clients
+ */
+@Component
+@Order(3)
+class CoinMarketCap extends ExchangeRateProvider {
+
+    public CoinMarketCap() {
+        super("CMC", "coinmarketcap", Duration.ofMinutes(5)); // large data structure, so don't request it too often
+    }
+
+    /**
+     * Returns an empty Set for the CoinMarketCap price provider.
+     * Price data of CMC provider is not used in the client anymore, except for the last update timestamp.
+     *
+     * @return Empty Set
+     */
+    @Override
+    public Set<ExchangeRate> doGet() {
+
+        return Collections.emptySet();
+    }
+}

pricenode/src/main/java/bisq/price/spot/providers/Poloniex.java

@@ -42,7 +42,7 @@
 import java.util.stream.Stream;
 
 @Component
-@Order(3)
+@Order(4)
 class Poloniex extends ExchangeRateProvider {
 
     private final RestTemplate restTemplate = new RestTemplate();