PeerGroup: don't try and trigger connections during shutdown

core/src/main/java/org/bitcoinj/core/PeerGroup.java

@@ -507,6 +507,7 @@ public class PeerGroup implements TransactionBroadcaster {
 
     private void triggerConnections() {
         // Run on a background thread due to the need to potentially retry and back off in the background.
+        if (!executor.isShutdown())
             executor.execute(triggerConnectionsJob);
     }