Reordered code in Block.mint() to fix potential issue after mempow activates.

src/main/java/org/qortal/block/Block.java

@@ -368,16 +368,17 @@ public class Block {
 
 		// Fetch our list of online accounts
 		List<OnlineAccountData> onlineAccounts = OnlineAccountsManager.getInstance().getOnlineAccounts(onlineAccountsTimestamp);
-		if (onlineAccounts.isEmpty()) {
-			LOGGER.error("No online accounts - not even our own?");
-			return null;
-		}
 
 		// If mempow is active, remove any legacy accounts that are missing a nonce
 		if (timestamp >= BlockChain.getInstance().getOnlineAccountsMemoryPoWTimestamp()) {
 			onlineAccounts.removeIf(a -> a.getNonce() == null || a.getNonce() < 0);
 		}
 
+		if (onlineAccounts.isEmpty()) {
+			LOGGER.error("No online accounts - not even our own?");
+			return null;
+		}
+
 		// Load sorted list of reward share public keys into memory, so that the indexes can be obtained.
 		// This is up to 100x faster than querying each index separately. For 4150 reward share keys, it
 		// was taking around 5000ms to query individually, vs 50ms using this approach.