add logging when event wallet id is not present

src/main/java/com/sparrowwallet/sparrow/event/StorageEvent.java

@@ -1,12 +1,16 @@
 package com.sparrowwallet.sparrow.event;
 
 import com.sparrowwallet.sparrow.io.Config;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.io.File;
 import java.util.HashMap;
 import java.util.Map;
 
 public class StorageEvent extends TimedEvent {
+    private static final Logger log = LoggerFactory.getLogger(StorageEvent.class);
+
     private static boolean firstRunDone = false;
     private static final Map<String, Long> eventTime = new HashMap<>();
 
@@ -22,8 +26,10 @@ public class StorageEvent extends TimedEvent {
             eventTime.put(walletId, System.currentTimeMillis());
             timeMills = keyDerivationPeriod;
         } else if(action == Action.END) {
-            long start = eventTime.get(walletId);
-            if(firstRunDone) {
+            Long start = eventTime.get(walletId);
+            if(start == null) {
+                log.error("Could not find start event time for wallet id " + walletId);
+            } else if(firstRunDone) {
                 keyDerivationPeriod = (int)(System.currentTimeMillis() - start);
                 Config.get().setKeyDerivationPeriod(keyDerivationPeriod);
             }