updating the wallet tip before and after the chain download

Breeze/src/Breeze.Wallet/LightWalletSyncManager.cs

@@ -37,11 +37,18 @@ namespace Breeze.Wallet
         /// <inheritdoc />
         public async Task Initialize()
         {
+            this.walletTip = this.chain.GetBlock(this.walletManager.WalletTipHash);
+            if (this.walletTip == null)
+                throw new WalletException("the wallet tip was not found in the main chain");
+
             // get the chain headers. This needs to be up-to-date before we really do anything
             await this.WaitForChainDownloadAsync();
 
             // if there is no wallet created yet, the wallet tip is the chain tip.
-            this.walletTip = !this.walletManager.Wallets.Any() ? this.chain.Tip : this.chain.GetBlock(this.walletManager.WalletTipHash);
+            if (!this.walletManager.Wallets.Any())
+            {
+                this.walletTip = this.chain.Tip;
+            }            
 
             // subscribe to receiving blocks and transactions
             BlockSubscriber sub = new BlockSubscriber(this.signals.Blocks, new BlockObserver(this));