Add null check at getMyTradeLimit

Other small changes:
- Remove final from User as http-api branch will require to extend it.
- Add comment

core/src/main/java/bisq/core/offer/placeoffer/tasks/ValidateOffer.java

@@ -98,6 +98,7 @@ protected void run() {
                     "maxTradePeriod must be positive. maxTradePeriod=" + offer.getMaxTradePeriod());
             // TODO check upper and lower bounds for fiat
             // TODO check rest of new parameters
+            // TODO check for account age witness base tradeLimit is missing
 
             complete();
         } catch (Exception e) {

core/src/main/java/bisq/core/payment/AccountAgeWitnessService.java

@@ -260,9 +260,14 @@ public long getMyAccountAge(PaymentAccountPayload paymentAccountPayload) {
     }
 
     public long getMyTradeLimit(PaymentAccount paymentAccount, String currencyCode) {
-        final Optional<AccountAgeWitness> witnessOptional = Optional.of(getMyWitness(paymentAccount.getPaymentAccountPayload()));
-        return getTradeLimit(paymentAccount.getPaymentMethod()
-                .getMaxTradeLimitAsCoin(currencyCode), currencyCode, witnessOptional, new Date());
+        if (paymentAccount == null)
+            return 0;
+
+        Optional<AccountAgeWitness> witnessOptional = Optional.of(getMyWitness(paymentAccount.getPaymentAccountPayload()));
+        return getTradeLimit(paymentAccount.getPaymentMethod().getMaxTradeLimitAsCoin(currencyCode),
+                currencyCode,
+                witnessOptional,
+                new Date());
     }
 
 

core/src/main/java/bisq/core/user/User.java

@@ -63,7 +63,7 @@
  */
 @Slf4j
 @AllArgsConstructor
-public final class User implements PersistedDataHost {
+public class User implements PersistedDataHost {
     final private Storage<UserPayload> storage;
     final private KeyRing keyRing;